What is the best replacement for the stock Amal 625 for a Cota 123?

not sure what year model you have but the mikuni roundslide is probably your best choice, replacment parts are common, my '80's 123 is fitted with a kehin, I would have chosen the mikuni but already had the kehin in my parts stash, best improvment I ever made on the bike.

Edited December 10, 200916 yr by MotoVintage

I have fitted a Mikuni to my 1980 123, got it from allensperformance.co.uk

I put a Mikuni VM24SH round slide on my 123/172 and it is very reliable compared to the Amal.