The dual carb 500s won't fit, or so I'm told. You need to find a 79-81 engine for sure, not sure on the 82 engine though. XR of XL will work, just trans gearing is different. I have a 2nd gen, 200x based Nick, yours is the 185s based Nick. Lucky.
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