First steps :)

if you want to use the back, then you'll probably want a front wheel drive, I don't know how easy it will be to install on that bike, but staton's front wheel drive is the highest quality available, and well worth the cost FDF-245 Robin Subaru 24.5 cc 1.10 HP Four Cycle Engine Kit: Staton Inc
You'd need to make some creative front mounts, but if you have skill, or know someone who has skill it is completely doable

The link I gave you will drive ANY bike on the front wheel. It mounts to the front and has a friction roller that makes contact with the tire and moves the bike forward. Friction drive is probably one of the most reliable systems available. You will NOT be going really fast, but the system in the link I gave you will go any day you want it to and won't take a lot of tuning. A rear mounted engine is only made for the rear of the bike, and one of the in-frame chinese 2 stroke engines would be much more work to mount to the front than just buying the mini 4 stroke friction drive. The other thing is that the robin subaru engine on the friction drive kit is EXTREMELY high quality, the chinese 2 stroke motors, at least in my experience are such low quality that they require one major improvement each week for a few months until they are perfect. This, in the end, will cost you much more money and time than getting a kit made for the front wheel. I hope this helps. Also, the man at Staton is very helpful, he's maybe a bit gruff but he knows what he's doing, don't be afraid to ask him questions about his products because he will make sure your project goes right

no problem, can't wait to see what you do