Don't do it. The engine case is aluminum, the bike frame is probably steel. Even if the bike is aluminum welding the engine to the frame is a bad idea for many reasons.
There are alternatives. Take some time and do a little research here, use the search feature, keywords 'broken mount' and see what others have done.
http://www.google.com/cse?cx=partne...gsc.tab=0&gsc.q=broken motor mount&gsc.page=1
In the mean time do not ride the bike using the engine with no front mount. The engine could break loose and bind the chain which could cause you injury and further damage to the engine.
Tom