Ya 19mm kinda too big for stock cylinder anyway let alone 21mm.
Do your homework! A stock high torque 2HP cylinder has a round intake hole 14 maybe 15 MM so why would you even want to put that carburetor or any 21mm square intake on.
Even if you could get to 21 mm carburetor to work running really tiny jets and dremeling the cylinder intake hole 21mm Square you still would lose a lot of low-end. A 14 mm Bing maybe 15mm Bing is best to use with your setup anyway. A Proma pipe tops out quick so soon as you 21 mm carburetor would start into the powerband your pipe would die out where as a 14-15mm carburetor when jetted correctly will give you good power throughout the range