The stock bolts work but are a tiny bit short IMO. Be careful though, if you cut new ones and they are too long they hit the crank counter balance which is right where the bolt ends. Don't worry though, you'll know right away because the engine will be totally locked up. You only need a couple more mm than stock the bolts. The stack is about 5mm thick plus maybe a mm for a lock washer but the stack and washer will compress a mm or two. Use a little lock tite and the stock ones will be fine.

If your only issue is fuel/oil getting all over the inside of the engine box you can put a balsa sheet over the inside of your firewall with thin CA. It's super easy to do. Look at this picture and you can see where I had the balsa sheet. All you have to do is hold it in position with your left hand and start to apply thin CA with a dropper with your right hand (through the hole in the firewall) where it comes in contact with the inside firewall. Just make sure you have enough room behind the air inlet of the carb to keep good air flow. With the balsa sheet on the inside firewall it gives you some extra space.