You are very optimistic -
"a good cylinder for this bike? That, a bore and two pistons, and this thing will be on the road."
I don't think it's going to be that easy. You'll need to pull the engine completely out and take the cases apart. That entire crank assembly needs to be taken apart and at least new crank seals installed, but my guess is you'll need new needle bearings and maybe a crank pin or two (Bill Bune Enterprises did my GT750 for $230). Exposure to any elements will rust/pit those parts in days. Yours has definitely been exposed. All those pieces are still available from Suzuki and not bad in price. You'll need new transmission seals and that's a great time to check for shift drum wear, clutch replacement, and dog and fork condition. The GT750 triple and this Titan twin are the same engine internally - using the same rods, pistons, and pins, so parts are everywhere.
Don't get discouraged, you've got a lot of bike there - I've restored much, much worse.
Regards,
Gordon