1972-75 engine, big bore 88mm?

Since you have a good one go with the Mikuni. If I'm not mistaken the 501 Maico of that era had a 38mm so I would think the 38mm Mikuni would be perfect. To large a carb and the air velocity is reduced possibly rendering the carb problematic.

Turns out the wrist /gudgeon pin is worn unevenly so will need changing. He found a new bearing for it but not the pin itself yet.

Any tips on what engines had the same 20mm size pin? It measured: Outer diam. is 19,92 mm, inner 13, length 70,75.

I purchased one of these for my 73 450 when replacing the small end bearing and it fit although a couple mm shorter than stock.
https://www.ebay.com/itm/CONNECTION...639378&hash=item3f84bb41c0:g:exEAAOSwUSZZlhPQ

Thanks for the link!

Found a NOS pin for a 450 engine at a local dealer so problem sorted!