Best bet, IMO, would be to get a Lifan MC type engine. They are "clones" (copies of Honda designs) and usually very reasonably priced. I say "usually" because sometimes the prices can double or even triple (may sound crazy but it's true, I check prices often). I assume they are in short supply when the prices jump up. I would like to put the 9,500 rpm, 11.4 hp, 4-speed, 125 in a small bike and/or the 8,500 rpm, 16 hp, 5-speed, 200 in large bike (MM 212 Pro Series preferably); I need to move to a more rural location where I could enjoy such things, no fun to build something just to look at it (might be for a little while, but then you want to see what it can do!). I've been through that before when I was building custom handguns years ago, it was a hassle packing up gear and going someplace to get in some shooting.
This Lifan 125 is usually around $330 + $50 shipping, I consider it a great bargain at that price. It's complete with carburetor, wiring harness, etc. You need to be careful ordering one because they are also sold in less complete form.
http://rover.ebay.com/rover/1/711-5...0001&campid=5335845805&icep_item=380151383562
This Lifan 200 is usually around $550 + $100 shipping (they were over $1,600 a week or two ago!), also a bargain at that price, IMO. This one is complete but the 200 is also sold in less complete form
http://rover.ebay.com/rover/1/711-5...0001&campid=5335845805&icep_item=160982683885