Personally I wouldn't go anywhere near that starter button until new belts were on there. If they are going to snap, they will most probably snap during the first few revolutions.
Don't chance it. Even if you can't get it to run, you will still have a non runner to sell, but a bike with a wrecked engine is not worth very much at all!! You will get the price of the belts back if you still have a working engine at the end of the day.
Once you're ready to fire up, you might want to prime the carbs with fuel before cranking, as they do take a very long time to fill. You might even flatten the battery trying to pump it through as the pump is worked by vacuum created by the turning engine.
Unfortunately the vacuum pump and tap don't fare well with age, so they may need attention a little further down the line. There's a relevant thread not far back, I will try and find it unless someone links to it before me..
Found it..
http://www.ukmonster.co.uk/monster/s...ad.php?t=58033 Some tips on there about how to get fuel into the carbs, spinning it up with the plugs out is a good tip too.
Re the tyres.. They might look good, but the rubber compound breaks down and is considered by many to be unsafe after 5 years from manufacture date. Again don't risk it, they will let you down with very poor grip and unpredictability especially in the cold!