Jason Aguilar ran a RS with a 450 out here for awhile. It would break parts from the vibration, exhaust pipes, radiator, etc. Would also wear out the cushes for the rear wheel very often and if not changed would wallow out the rim. Tom Aquino built one in a TZ250 chassis and that one was much better suited for an adult.
I built one last winter. 450X motor (w/ R top end) in a RS125 chassis. Just under 200lbs wet. It was a lot of work and I'm still working out some bugs but it rips. More pics here: https://photos.app.goo.gl/bsrByKuUUj4L2QpeA
No, it wouldn't, a purpose built race bike running as designed would be a much better machine than a collection of parts from different bikes put together like the SuperSingles. They are fun, though, and mine made a better track bike than the Moriwaki because I had more power exiting the corners and it wasn't so tiny compared to the other bikes, plus being a heck of a conversation piece. It only weighed 240 lbs and handled well, much more forgiving than the Moriwaki - not as razor sharp in the corners, but not as reactive to body position. The NSF250r is another option - comes ready to race, fuel injected, better transmission than the Moriwaki, has design improvements, better all around except for price and the fact that it doesn't have any kind of starter so you need a roller starter for it.
There is a brand new one that has been sitting in the dealer showroom for YEARS. Plain white with a ton of dust on it. No clue what they are asking for it. Hinderer Honda in Heath, OH correction: they list it as 2008 "Used" - but I don't remember seeing any wear on the tires. https://www.hindererhondapowerstore...a-Motorcycle-Scooter-MD250H8-7843661?ref=list
+1 on this, it's worth a try. Keep in mind any used Moriwaki you find may have had a hard life. Mine had been ridden by multiple racer kids and worked on by racer dads and a variety of mechanics over the years. I still don't know if all the hoses are routed properly or all the spacers and washers present that should be.:/ Starting with a fresh one in factory condition would be a nice benefit and probably worth a few grand.
Matt, I still have two Kaw Ninja 250's for sale. Curious, why are you not looking at the Aprilla RS250SP for Ian? Newer machine, nice race bits (suspension, brakes, exhaust, etc) too.
While the Aprilia seems to be a good bike. Its 12k no class for it to be competitive in that I can think of and its still 230lbs. Talking with Reuben, I think he may be right and I need to think of the weight of the machine Ian will be riding and if it gets out of control and can physically control it as safe as he can. Now if there is a T.K.D. bike sponsorship of a ninja 250 well that changes the game.. Honestly Tim, I dont know which direction we are going to go in. There is about 10 different directions to choose from. This damn kid, I'm just put him in dance class and call it good. Lol
You just missed out on the EX250 I sold at Nelson. . Based on the weight concern, that probably rules out the KTM 390 which I know you weren't too keen on anyway. There are a pair of RS125's for sale in the AHRMA group. https://www.facebook.com/groups/1590179347894723/permalink/2761051264140853
I think your next step is to figure out which chassis fits him and how fast he's growing - then get whatever is affordable and just a hair large on him. He'll have fun and get better riding any of them - as much as I personally love the 2 and 4 stroke GP machines, I'd suggest the 300/321 or 400, get one already prepped and then all you have to screw with is suspension and tires. It'll make both you have a better weekend at the track. If you truly want to tinker then any of the GP bikes will work for him too.
Just say no! And if you put a 450 in one, it won't go any faster. It will, however, vibrate to pieces twice as fast, which is saying something.