If your local machine shop cannot or will not line-bore the cam bearings to fit available bearings,find a shop online that can think outside of the box and fix your head. I know that this can be done,as some aluminum heads without cam bearings have been bored to accept bearings,either standard shell bearings like main and rod bearings or solid bearings indexed with a dowel. The solid bushings can be machined from a piece of bronze to fit into the head.either way should get you back on the road for a long time. Good luck with your project,and post results so that others in your situation have an alternative to buying a possibly impossible to find,and expensive head. This was done with many ancient motors with pour in place babbit bearings so that one could use shells instead of having to pour metal and ream to fit the crankshaft.