As I said above, I did an engine swap on a 12G the "G" means gear drive which is what the "Vari-Speed" is From what I understand, the 10G and 12G are basically the same tractor except 10HP Tecumseh vs a 12HP Tecumseh.
If it is the 10G or Vari-Speed tractor you are talking about, there may be some issues getting the drive pulley for the engine. The Vari-Speed belt pulley is specific to that type of belt and you may need an original engine pulley. That pulley may be 1-1/8" shaft size and most replacement engines will probably be 1". I bought a 20 dollar "adapter sleeve and key" to make my original 1-1/8" pulley fit the 1" shaft engine I re-powered my 12G with. From what I also understand the pulley setup isn't quite so complicated on a hydra-speed tractor.
The first pic is the shaft sleeve and the second pic is the sleeve installed with the pulley.