GK you should be able to find a used one... since people who have made the 70cc upgrade will have an extra engine kicking around.
I'd mention the bike and year and search around for it. You could probably find one for $20.
If it was any other bolt I think the JB weld and epoxy would work, but because its under 100psi I think you will get a little leakage, and a big loss in performance.
Or look on the bright side, this is a great excuse to get a 70cc kit
Matt