newbie question

I have a Troy Bilt TB6042XP (model) 41ADZ4PC066 4 stroke 25cc string trimmer that is 12 years old. I know it has had the wrong oil put in it at least once (for a 2 stroke). It leaks oil, even when laying flat.

I am a very proficient DIYer, extensive electrical, plumbing, carpentry experience, but mechanical has always been a knowledge gap for me, and I'd like to use this as a learning tool. I am having issues finding a "replacement" engine, I think because I lack the knowledge to know what to look for. I would also like to rebuild the existing one, just to learn how to do it.
 
My questions:
1. Any info (part #, website, etc) for a replacement engine (will reuse all other existing components)
2. A list of what parts I should get to rebuild the existing (gaskets, piston rings, etc).

Any help is appreciated. Link to exploded parts view is below. Thank you!