I had a CR500 and it was an original Tecumseh bike with jack shaft. Most of them were actually because the Tejunksie engines are a butt load cheaper and far less suicidal to attach to 6 inch wheels... It sounds to me like you MOST DEFINITELY want to just use whatever engine is available to you. That Hodaka engine is going to be hard to find, cost you $350 IF IT RUNS, THEN the kick starter gear is going to be stripped out so you have to push start it because those were always the weak link.. THEN once ALL OF THAT stuff finally works out, your kid is going to be killed on the thing because #1 those engines are wayyy WAY too powerful for those little bikes #2 it uses a manual clutch like any motorcycle, and #3 In first gear they have like creeper speed and a 2 cycle engine that runs VERY strong.... They literally will flip you over backwards at lightning speed and you almost can't stop them.. ANOTHER scary design flaw on ANY of those mini choppers is the fact that you sit directly over the rear wheel. WHEELIES happen with very little effort.... I ride them like I stole it and expect to crash, but I'm here to tell you just a simple hopped up Briggs 5 horse wound for sound is WAY WAY scary enough as it is.... I loved my CR500 but I can honestly say that I have never in my 200 motor vehicles, EVER owned one that scared me as bad as that chopper did... There have been more than one instance when my heart was in my throat because I could feel the back of my being WHIP LASHED to the asphalt below. Luckily It never got that far. They are cool ass little bikes, very valuable as far as vintage minis go, they honestly drive very well if you are just driving it around. BUT!!!!!!! and I mean THE BIG BUT is they are VERY VERY prone to wheelies and you are about strapped in and can't get away from it when it does, which doesn't matter anyways because it happens so hahahahain fast you haven't got the time to react... I watched ole Doofy Phil test drive his Ruttman chopper out here after taking the governor off and adding a carburetor that actually runs on a STOCK TECUMSEH 4 horse... watched him whip a 180 and flip over backwards with the engine going crazy and throwing dirt, and this guy is like 40 and should know better... It's more a matter of hoping and praying the engine doesn't have the RPM's built up to get you all the way over before you can shut it down.... If you are building a bike for an 11 year old kid DEFINITELY just use some little 3 horse Tecumseh or whatever goes in there easily. Briggs engines most of them require a different intake and carb system because the gas tank is in the way as are the bigger Tecumseh 5 and 6 horse motors.. But any old 3 horse, and proper gearing is your best bet... You can always have an on going search for that elusive E Code Hodaka strictly made for aftermarket sales, to buy as the final part of the restoration YEARS from now when the kid is big enough and experienced enough to HANDLE a 100 dirt bike engine wound to 10,000 RPM's and dumping the clutch in second gear...... If you struggle to get a perfect Hodaka CR500 put together for his birthday...... the end result is going to be an 11 year old kid with a head splattered all over the ground and a torn up sissybar and seat on a $3,500 dollar mini bike... Tecumseh 3 horse geared for 30 MPH and the governor working properly is the only way to go on that bike... I have a couple nice Briggs 2 horse motors from the 60's that would work good I'd sell ya but then your cool Harley oil tank would have no function. I'd say find some 25 dollar craigslist Tecumseh 2.5 horse that runs great off from a sidewalk edger and forget that Hodaka engine... That thing is a hard to find, guaranteed kid killer... :sadcrying4: