The GX120 is not too powerful in my opinion, but the big Preddy is too much motor for the bike and won't fit, or so I have been told. I've had the same frame (53 Hornet) with a 99 Preddy and it was about right. Not quite enough for mountain climbing (long, steep hills) so I sold it.
I'm getting things together for something more like a Whizzer and had thought about a five horse Briggs, but have decided to go with a governed 147cc Jacobsen 2 stroke (1970's engine which powered a snow blower, made in Wisconsin). I believe that will be around the power range of a Whizzer and is about the upper limit for my 51 cantilever Schwinn frame I think. Even with that engine I'll be watching my frame closely and am building wheels with Atom drum brakes and 11 gauge spokes on original Schwinn rims, Moped suspension fork.
Your bike looks good, the new seat and gas tank will make it better yet. That's one of the cool things with building your own bike... you can change things as you go, make it the color you like, give it the engine you want, etc. and then change your mind and give it another do over.
SB
got rid of the seat and the springer fork, bought a monarch double springer fork and a saddle from j&p cycles. have a 48t pirate cycle sprocket with the adapter. also have a 79cc HF greyhound but am thinking about maybe going bigger.
