Jam – How to Make Pumpkin Preserve: Step-by-Step Guide

The closest in terms of taste an texture would be another winter squash such as butternut, acorn, hubbard, etc. (As has already been mentioned.)

However, anything with a similar texture would work. If you don't care so much about the taste, you could substitute a tuber such as potato, turnip, rutabaga, carrot, parsnip, celery root, or parsley root.

Sweet potatoes or yams would be closer in taste, but might be mushier than you want.

I'd suggest experimenting with any single or combination of the above and see what flavor you like best.

Based on your description and some of the feedback from comments, I think the most likely candidates for your mystery gourd are white pumpkins of the Cotton Candy or Valenciano varieties, possibly also Lumina or Silver Moon (though those tend to be flatter and larger than you describe). Here's a link with some further comparative description.

The dark color of the flesh that you describe was more likely a product of the growing season than anything else, though you might have some luck asking farmers at local pumpkin patches what specifically could cause that result. How you cooked the mash could very well have contributed too. Here's an excellent post from the community blog with instructions for producing quality, richly-flavored "orange goo".

Should you be completely unable to find a similar specimen, you may need to consider substitution. Possibly the best one I can think of is butternut squash. This has a very rich flavor, quite similar to pumpkin, and it gets richer as it ages. Look for ones with dark skin - the darker the outside, the darker and sweeter the inside will be as well.