Back from the summer job, had a blast, but now its time to ride yyeeeaaahhh.... about that. Par for the course there's things to be done on the bike.

The gasket under the magneto cover is old and leaking oil, so I've got it apart and the new gasket ready. However, the old one is crusty and stuck on there big time. Any suggestions on how to get it off cleanly? I tried an xacto, but it scored the aluminum. Will a solvent work? How can I prevent this from happening to the new gasket?

Many thanks
-Fin

Funny - I just had that replaced at the dealer.
Too bad I didn't ask any questions about how they got the thing off but the service manager just said it took awhile to 'scrape' it off. That's all he said. 

They have these things called gasket scrapers; they're a handle that you can stick a razor blade into.  It gives you a good amount of leverage on the blade, and as long as you keep the blade perpendicular to the gasket surface you shouldn't scratch it.

Yeah, the gasket was taking quite a bit of scraping until I used a random orbit sander errr... Hopefully I didnt mess up the seal but its damn clean. Washed the cover out with some gas to get dust out and let it evaporate. Does the new seal just sit in place? no sealant or anything? and do I cut off the excess seal?

Your method was the same I used, except I used a sanding block and a scraper. Minor scoring/nicks are not a big concern. The gasket will seat into them.

Put some axle grease -- thin coat -- on both sides of the gasket. It will help hold it in place while the cover goes on and make it easier to remove the next time.

You could use some Permatex gasket stuff, but the grease is an ancient trick from cars and motorcycle wrenchers.

Tighten the bolts to spec, in an alternating pattern, and you should be good to go. While you are in there, clean the gunk out from around the drive sprocket and cover. It can get maxi-nasty. 

This is the easiest and quickest way to remove hardened gasket material, I have a can in the garage and use it whenever I need to replace a stuck gasket. Its quite expensive but lasts a long time, I "borrowed" it from work and keep forgetting to return it 
http://68.72.74.113/PRODUCTS/790.htm
There are similar products available from other companies. It sprays on like shaving foam, you go have a coffee/tea/beer/joint etc and in 15mins the old gasket has completely dissolved and the remenants just wipe off. Just be careful and avoid getting any on painted surfaces. We use it SAVE labour time when overhauling industrial gearboxes and pump and pipework installations. If your dealer says he scraped the old one off treat the comment with a pinch of salt, they all know about this stuff and and I would be surprised if they dont use it, they charge by the hour remember and its easy to say it took 30 mins to scrape the old gasket off and bump the bill up.