What are you planning on doing? I know with certain machines, you mainly want to make the intake valve larger, not the exhaust. With a larger intake and some professional port work, you can really wake a 4 stroke up. You want to move more air and fuel into the head, and the larger volume of air, will also force the exhaust out quicker. So that is why you need some exhaust porting. Port matching your intake to your head is a big help and some polishing of your intake and exhaust never hurts either. Don't want to give away too many of the secrets. Sometimes, the valves you use won't even be for an atc, but you will have to mod them somewhat to fit. Welcome to the world of performance for 25+ year old machines. And once you start talking valves, you may as well start looking into stiffer springs or double wound springs and titanium spring keepers and retainers. $ can add up quick to go fast! Better roller bearing cams as well as slightly higher compressions and better grade fuels help also. How $erious are you looking to get? Of course the more you build your engine, the less reliability, so you have to know where you want your trade off to end.
Nicholson 500x (Project)
Nicholson 185s
Nicholson 110 (That takes an atc70 tank)
Ascott 500 head/350x (Project)
Homebuilt Racer Chromoly Suspension 110 (Ragin Runt)
PK Racing Suspension 70
A+ Inc Suspension 70
AWS Aluminum 90 frame
Hi Performance ATC Suspension frame rd350
Hi Performance ATC Suspension frame 90