The easiest thing is to get a 400-4 engine. It will bolt right in, but you will need a 400 clutch cable since the actuation is very different. There are advantages to this motor, it has a six ratio transmission and you can get a 466 overbore kit either for around $800.00 from the 400fourstore.com fellow or maybe much cheaper on ebay sometimes... some guy in Japan has been selling them for under $200.00 IIRC and I haven't seen any complaints about them. With the 466 kit plus a cam and some head work and such the 400 engine can be fairly potent (for a 70's 400). Carillo will make you a set of con rods and leave change (maybe) from $2000.00 if you want to get really silly: with these plus a 466 kit, cam and headwork , a good header/exhaust and 26mm CRs you could be making some impressive (for a 70's 400...) power at 12000+ rpm. You can have the stock carbs bored out (making the odd shaped throat round) and improve the full throttle flow for good power up around 11000, the stock carbs limit you to about 10000.
The skinny rims, flexible frame, and bicycle size forks are still a problem though. Getting the chassis capable of handling the new power brings up a new set of costly mods.
There's just not much available for the 350 Four. The old Haynes manual for the smaller Honda fours (350-550cc I think) has a "performance improvement" section - one of the first paragraphs says basically "don't waste your time and money trying to hop up a 350 Four".
Overall a flogged out older 600 sportbike with a few bucks spent refreshing it will give you a much more capable machine. Not as much class, of course! There's also the Honda super four 400 (is this still available new in Japan?)... I've seen one in Toronto so there's some way of getting these things. Other markets get some really nice small displacement bikes.
