Also, if you start checking into it, the 80-81 125s are alot different than the 82 and later 125s. Very few parts are interchangable between the early and later model 125s. The early 125s have a smaller clutch basket than the later 175s so you may notice slipping of your clutches with that 175 top end. The early shifter link assemblies are much punier than the later shift assemblies. The frames are different, even though you can put the early or later engines in either frame. You can't put a suspension 175 kit on an early 125 frame without doing some welding. The early and the later rear axle bearing carriers won't swap out because they are different. The handlebar off/on switches are different, well at least between the 175 and 125. And there are a bunch more differences I can't remember. The 82 and later 125s and 175s are better made than the 80-81 125 in my opinion. And I've had an 80 125 and now a 82 yt175.
Nicholson 500x (Project)
Nicholson 185s
Nicholson 110 (That takes an atc70 tank)
Ascott 500 head/350x (Project)
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PK Racing Suspension 70
A+ Inc Suspension 70
AWS Aluminum 90 frame
Hi Performance ATC Suspension frame rd350
Hi Performance ATC Suspension frame 90