There are machine shops that say they will do Corvair heads, but ask them when they last did a pair and you get DUH!!! In the last decade two machine shops closed in my area and another has scaled back to just "minor" work. A number of reasons, but today most hobby guys just install a crate engine from the big manufactures and use a modern new transmissions.
I ran into this with the Corvair pressure plate. I got dozens of leads and EVERY SHOP I CALLED said "Oh we can't replace the diaphragm spring as they are NOT available. We test them and if they are bad you have to find another pressure plate"!!!! Someone posted on the CORSA forum that Clark's no longer has the rebuilt pressure plates with the near stock 1,500lb pressure plate diaphragm. They only have the 1,800lb pressure plate diaphragm and it requires some good leg muscles to use based on responses to the post. No Clark's will not sell the pressure plate diaphragm separate, you can only get it by buying a rebuilt pressure plate from them. Can't say I blame them, but even that option seems to be going away.