Considering the limited frame space on these, you do need a small engine that can make decent power output so this is definitely best done with a 2 stroke engine, tho there are a few 4 strokes out there that can fit the bill... I got all the billet mounts and such to install a CG engine but I also got an older Morini engine that's looking for a home and I may go that route since it can put out triple the power a CG can and it takes up close to the same amount of space, AND it has a built in CVT drive which makes the deal even sweeter. If I decide to use the Morini it's going to take some fab work to get it in there, but if I use a CG I can just assemble one of the Dax bottom ends I got in my stock room since the Super Rat I originally built for it won't bolt to the billet mounts without modification.
If you want to go with a Morini engine you can buy the weld in mount plates from Treatland then the engine will just bolt right in or out, and you can get the CVT version of the engine from them if they have it in stock for about $400, it puts out 3.9hp in stock trim, but it can put out closer to 10hp with a better reed box and bigger carb.
Thatxdax.com also has an 8.5hp Morini clone for under $500, that one is single speed but put a Tomar clutch on it and tune it to your liking it would be an excellent powerplant. Dax also sells KTM clone engines that are good and put out 9.5hp but you'll be on your own for coming up with how to mount it, and you'd get better help for souping up the Morini if you chose to get that one.
If you want to go with a CG engine I'd recommend the billet mounts sold at Pedalchopper.com, they're a bit expensive, but they're by far the nicest mounts you can get for the Stingray with a CG engine. He does sell the cradle type mount if your budget is tight and you're not going for show bike quality or looks. You should gusset up the frame too once you decide which engine or mount set you're going to use, but get the engine, the mounts, and all before you start beefing up the frame so you don't add gussets where a bracket might need to go later. I'm leaving ine bare metal until I'm sure which engine I'm going to use just for that reason, then going to install the engine and mounting system on the bare frame to take measurements, then once I know what kind of free space I got I can go in and add some gussets and strengtheners where needed without having to worry about having to cut one out later to fit a mount etc...
I'm sure you figured out by now that I'm all about using the 2 stroke engines due to their simplicity, their power output, and they're real easy to considerably increase the power output without breaking your wallet to shreds if you need a little, or a lot more out of your engine.
