I've been reading up on Raggi's Lamentations of the Flame Princess, and was considering introducing it to my players using one of the adventures but using 1e/OSRIC rules. Would this break the whole concept? Would using 0e/LL rules without the LOTFP modifications?
[RPG] How connected are the LOTFP rules with the adventures
adnd-1econversionlamentations-of-the-flame-princesspublished-adventures
Related Solutions
Here are some original old school D&D modules that should be easy to find and not cost you an arm or a leg if you buy them online:
I1 The Forbidden City (TSR, 1980): Old school AD&D sandbox setting in a "lost" jungle city inhabited by snakemen (yuan ti), frogmen (bullywugs), and lots of other weird and dangerous creatures. Lots of room for development here and no two adventures will play alike (due to multiple adventure paths and foes).
B4 The Lost City (TSR): This old school original D&D module consists of a pyramid buried in the desert sands which holds various degenerate groups battling each other for control of the underground empire.
X1 Isle of Dread (TSR): Lost island sandbox setting with lots of different adventure ideas
WG4 Lost Temple of Tharizdun (TSR): written by the master himself, EGG, this AD&D adventure takes place in a long abandoned temple to a dark god hidden in a mountain valley. Initial conflict is against monsters who now inhabit the structure, but if the party delves deep enough they will run into Things Best Left Undisturbed.....!
Axe of the Dwarvish Lords (TSR): This AD&D supermodule written in the 90s takes place in an long abandoned Dwarvish citadel now inhabited by an army of goblins.
Gates of Firestorm Peak (TSR): This 90s AD&D module takes place in a weird, otherworldly mountain that has connections to a "Far Realm" of madness and insanity. Sounds like it would fit right in with Raggi's stuff!
Not as easy to acquire are the classic Judges Guild modules Caverns of Thracia and Dark Tower (however, the 3.5 reprints should be a lot easier to find and are backwards compatible). Both are great and concern ancient, underground empires with lots of evil things lurking about.
You also need to discuss expectations if you haven't already
If you just change the mechanics to better fit the style of play that a highly lethal old-school module is written for, you need to discuss the expectations of this style of play with your players too. If you don't talk about these expectations, all that your players will see is that you've changed the system and made them less powerful apparently for no reason. I would highly recommend having your players read a quick primer on old school play; if you can get past the bias inherent in the document, it's very helpful. Even if you don't have your players read the primer, you can still state the main points and present the reasoning behind you've changed the system. Making sure players know that build is less important, that death is frequent due to lack of caution, and that player skill is rewarded is going to be necessary to support the style of play you want here. Other than this...
Running an LotFP module with 5e is perfectly viable with some conversion work
As long as you convert the module's mechanics over, running Tower of the Stargazer should work fine. The main issues would be hit point inflation (which is sounds like you've covered); the Perception and Investigation skills (which it also sounds like you've removed), and Saving Throw DCs. I'm assuming (based on your question) that you feel comfortable making mechanical changes to both the system and the module, so I don't think this will be a problem for you. One point of caution I'd like to advise on, however, is length of time spent to make a character.
Avoid Lengthy Character Generation
The general expectation of a game like LoTFP is that characters die frequently, and thus, must be created frequently. You'll want to keep the time a player spends out of play due to a dead character as low as possible. A LoTFP character can be created somewhat more quickly than a 5E character, so I'd advise making some of your system changes reflect this. Particularly; I'd say that you should have a list of cantrips and 1st level spells available as handouts for your spellcasting characters so selection goes more quickly, that you either enforce rolled stats or use a standard array for scores to not spend a lot of time on point buy, and that you don't use feats and skills (which it sounds like you've already done). This is just a very quick and dirty method of reducing time it takes to create a new 5e character, but hopefully it will work for you.
Best Answer
If using a 1e/OSRIC group with an LotFP adventure, no modifications should be needed. The challenges are often not connected to character stats. The adventures should work equally well with any traditional ruleset.
If using LotFP characters with 1e/OSRIC modules, since those are usually more combat-heavy I would make sure the LotFP characters are one level higher than what the module recommends. For example, if an OSRIC module states it's for adventurers levels 4 - 7, I'd make sure the LotFP group is level 5 - 8.
I would make the same recommendation if using Basic Fantasy, Labyrinth Lord, or Swords & Wizardry characters with a 1e/OSRIC module, because 1e characters are generally built tougher, both due to bigger dice used for hit points and because of greater benefits for higher ability scores and a greater chance of having those higher scores.