Saw this on eBay. It looks pretty ratty, but should clean up nicely. Only 7 hours to go on the auction. The seller has heads and some other engine parts listed, too.
http://www.ebay.com/itm/75-76-MUSTA...ash=item3f709e43ff:g:~zYAAOSwnHZYQjaK&vxp=mtr
http://www.ebay.com/itm/75-76-MUSTA...ash=item3f709e43ff:g:~zYAAOSwnHZYQjaK&vxp=mtr