i have a McCulloch weedwacker engine and i want to cut down on the weight. Do you think that i could grind down the cooling fins?

You can trim the vanes on the flywheel ( watch the balance! ).
Leave the cooling fins on the barrel/head well alone!

I have a local guy that charges me $5 a flywheel to turn them on a lath. Check around and see if you can find someone to do it for ya. I'd pay $15 or $20 for the result but dont tell the guy that does it for me.

I second with Mr. Piper. You have to keep the thing balanced. If you ruin the flywheel a new one will cost about half as much as the engine. If you cant find a local machine shop call Peter Faith at Wacker Engines. He machines it and balances it for you for $15 bucks.