you would have to measure the bore wear, could be just worn rings and a bore hone or at worse case, a rebore and new pistons/rings.
its complicated if you don't know what you are doing. I suggest you get a haynes manual and read it, then you'll know what's involed.
It's not an engine out job, just top end, new gaskets required (£20for top end). pistons will be he hardest to source I think +.75mm is the only ones you can get now and a specialist to rebore and hone the engine (who will need the pistons for measurement)
I woudn't recommend running the bike like that as you'll blow seals in the crankcase, Get a second opinion off someone who can actually see the bike in person, because I can only go off what you tell me on here.