Here's a link to some self sealing solid inserts:

http://www.mscdirect.com/product/05980594?fromRR=Y

Also, you could try JB welding in the helicoils as well. If everything is clean it should hold fine, just let it cure a couple days. It seems to handle heat real well. I JB welded a check valve onto a smog tube once as a temp fix to get my old truck to pass smog, it was hard as a rock and still holding 6 years later!