Being only an 84 vintage, short of something serious like a block, everything you might need will be available. I have sourced new parts for 70's models recently through my local dealer. Here is a link to the parts manual, direct from briggs australia
If you dont have spark I would suspect coil, as it is not the original one anyway. Being 84 briggs were long finished with points, and you're coil is an old points coil with a briggs Magnetron kit retrofitted. The one on youre 12hp should bolt straight on.
Just be aware the 12 is more different to you're 8 that it would seem. They are physically bigger, the carb i'm pretty sure is a bit different too. So things like the air filter housing probably wont just bolt on, unfortunately.
If it sounds clunky it could just be rings, or it may be more serious like a big end or (typically due to a lack of service) play in the synchros, which make the engine a write off.
