Ok so your numbers match up with a gx200 gasket I got sitting here. So yes the 212 heads will fit on the 120 at least by the bolts. I’d imagine a decent shave of at least .060” is needed to get the bore in the head down to piston size. Also have to see if the valves will hit the cylinder wall. Maybe the Hemi head valve angle will allow valves to be better clearanced for the cylinder. This opens up the possibility of easily building a hot rod 120cc.
My opinion is, a hotrodded 120 is the best size for a bicycle...