It's an '82 XS400SJ. There were both SOHC and DOHC bikes produced in '82.
As for the seat.... Nothing will just "fit", especially with the hack job. You'll need to come up with custom mounts for anything you find.
I'd measure the bike to determine the total length of seat that will physically fit, find one that matches that length, and fabricate mounts when it arrives.
Looks like it'll be tricky with the way the fender has been kicked up.