i will say this jawnn, anythings possible, all you have to do is spend time surfing around and you find fellow bikers coming up with amazing drive systems of their own accord or buy prebuilt kits and modify them to there needs.
It just depends on your own level of engineering ability mine been 1 on a scale of 1 to 10 thats why i stick to prebuilt kits, with the name brand reliable engines. Start sketching up ideas of the system you envision, get some measurements, engine size etc and go from there,
There are plenty of members in here that will give you the heads up on your design and lead you in the right direction etc. etc. but the more you post with diagrams, pics of your ideas, the more people will be able to help you.
my first suguestion for you would be to get an idea of the engine you want to use and work from there designing your drivetrain. the mits 43cc (2 stroke) titian or honda 50cc (4 strokes)should all work comfortably for you with the right gear ratios.
So if you think you can fit the engine under or behind the seat then you should be work out a system to power your rig this way.