Firestone

Hi, I'm a newbie to biking and this forum so, firstly hi to all. I've bought an '88 YX600 Radian as my first ride which on the whole is pretty good for the year however I had to replace the clutch plate gasket as there wasn't one - only sealant which stopped the oil leak. I noticed there was a distinct smell of petrol in the oil. The bike has not been extensively used over the last few years and I was told the carb diaphram may be letting petrol down through the piston rings into the sump?? Could anyone give me a bit of advice on this one. Thanks in advance, Paul.