Hte first engine I totally tore down and rebuilt was a BSA Gold Star, essentially the same as a Bullet. It's not terribly hard, but you will need a new Crankshaft, rod bearing and rod in an upgraded mode. these assemblys are available assembled, which is the way to go. I had to assemble my BSA Crank.
The rest of the stuff, main bearings, piston and such are going to be dependent on what shape the original parts are in. You need to tear the engine down and see what parts are ruined.
While you're doing this disassembly, take copious pictures so you can see/remember where everything goes! It'll help immensely. This means from when you start to take the engine out, through engine disassembly. You really don't want any parts left out when you think you're done!
Bare