Welcome to OSS
I take it being a 91 it's a oil cooled motor not air ?
You will find the best way is to build the bottom end and fit the positions to the con rods then slide the barrels onto the piston.
You can get piston ring clams but I've never got on with them.
But the srad had a (think) 32mm spindle or should I say much much bigger than the 20mm and I don't think you can get bearings to reduce down to the 20mm
Also had a 220mm brake disc not a 240mm
They are mix and match, you need to put up pictures of the numbering etc that's stamped on the wheels if they are both 20mm spindles I'd guess they will be ever b12 or RF9.
I've used the 750et gears in GS 850 and 1000 many times with good results.
Just read that people have used them in the 1100 and had a mate that used the 750et gears in his 1100/1170 et
But that's what he said