Greetings, Mr.Vango,
I hope you solve the problem(s) with your GV because I find the GV250 the perfect all around bike. Mine top out at 75 mph and are light enough for my 86 year old back to easily push around. I also like the retractable mirrors that allow me to squeeze through traffic in California, which is legal here. Mine have 12K and 22K miles on them and don't show any signs of old age yet. I didn't see black belt dust when I changed the belts and rollers, so perhaps the venting is good and this is normal for this model? I, too, had to replace the muffler on one of mine (the baffles were rattling around inside) but it didn't make any difference to the performance, and very little to the loudness. My GV's have a slightly loud bark even with good mufflers. I removed the lumbar supports, added a loud horn and a rear box, otherwise they are stock.
In addition to Ruffus's comprehensive list of possible solutions I would ask the seller if the bike had this lack of performance before it was parked and if not, assuming he's honest, then I'd look to all things fuel related. The three carbureted bikes I've rehabilitated after sitting more than a few months all had major fuel issues: rusty tank, clogged filter, gummed up carbs. I've had to re-clean a gummed up carb after failing on the first attempt. Did you soak it in carb cleaner overnight? Use high pressure air on passage ways? That varnish is tough to remove.
Good luck. Please keep us posted on your progress.
Stu