manifold carb gasket sizing for st50 engine?

hi there, i have st70 frame and st50 engine. i measured the gasketry between the manifold and carb. the hole in the middle is 18mm. is this right for a st50 engine/keihin carb? if so, where can i get an 18mm gasket? thanks. any other info appreciated.

outside dia. of the manifold between carb and engine is 29mm approx.

ps is a '72 st143xxx st70 a K1? why are a lot of the gasket sets and parts showing up as K2 late '70s? are the parts interchangeable?

First, you don't need to get the exact same gasket sizes.
If the hole is too small, simply enlarge it with a sharp knife.
If it's too wide, just use it as it is.
Again, there were quite a lot of different kinds and sizes of original carbs and manifolds, from 12mm to 20mm appr.