Adding split AC to a Dayton oil fired forced hot air furnace

Using epoxy to hold the glass itself in place would be a bad choice, as the thermal expansion of the glass is different than the surrounding materials - this is why a clip is used rather than a more secure solution. The clip allows movement.

If you can figure out how it was originally attached, and repair that (perhaps with the aid of a welding and/or machine shop then that would be the ideal solution.

However, using epoxy to hold wire or a metal clip of your own design in place that holds the glass in place would also work out well enough.

Note: If the item that was holding the glass in place was rusted/damaged enough that it no longer serves its function, chances are very good that the heat exchanger is also nearing the end of its life, or already has pin holes, which would allow combustion gasses to enter the building. Do a thorough inspection.

All gas furnaces require a flue to exhaust carbon monoxide outdoors, the reason modern units require a fan to do it is due to the circuitous design of modern heat exchangers that extract more heat from the unit before exhausting it outdoors, natural venting will no longer happen if the induced draft motor is not running, So much heat is removed these days they can be vented using PVC flue rather than metal and moisture condenses making an acidic condensate that is drained just like the AC condensate is. It is normal,if it were not so you would/ could die by operating the unit in that condition.