if your going to stuff an engine that big in there, why not look at a CRF250R? close to the same size but 2-3Xs the performance and is water cooled so you can run all day and not over heat.
for suspension, you can look at custom triples and I would get a metaltech custom swing arm made. would cost around $600-$650 and you can get in in any length you want. we are running a custom done one on the Baja trike that will use a 86 250r hub and brake assembly and bolt to the 350x chassis. Those guys do great work!
rear shock? talk to Milner, he can build what ever you want. Onformula1
take the stock 250r and get it modified with better springs, better piston and valve design, longer clevis for more travel, and re-anodized for strength and wear.
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Nearly every kind of Honda ATC (plus some custom ones
several Yami Quads (mostly custom built for MX racing)
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Ya actually don't talk to Milner.
85 Tri-Zinger 60
85 ATC250SX
86 ATC250SX
87 ATC250SX
02 XR650L conversion
84 ATC 480R
Wow! Thanks for all of the advice everyone. I really appreciate it. I think I'll just let the frame and swing arm go to a needy enthusiast and buy a complete 350X down the line. Unless I hit the Powerball. Then I'll have my TPC setup I've been drooling over for the past 2 months. Thanks again y'all!