Re: 4 hp honda
Is it a GX140 OHV engine? if so those are very good engines.
If so you will need a welder, a cutting torch, a good selection of hand & some power tools and the knowledge to fabricate, A mounting set up, a jackshaft if it would in the frame or you could build a platform mount that would set the engine over the rear wheel and then you could build the gear reduction system there on the engine mount and route the chain from there to your wheel sprocket..... or find you an old Comet Torque-a-Verter set up, that is what I plan to use on a bike in the future.
many things that can be done ith just about any engine if a person has the tools and the know how, it really depends on that.
Thats why most people buy these kits, very little fabrication to do to get them set up and going on a bike.
Good luck with what ever you do, hope you can build a great ride with that engine..!