You can try Bob Boumstein, P.O. Box 39085m Omaha, NE 68139. He deals in used engines and sometimes has a parts only engine around.

A new head at Tower is about $60.00, a new crankcase is probably more since it might have bearings installed, etc.

The valve cover off a Magnum 70 will fit on the OS but not much else will work. BTW the vavle cover at Hobby People for the Magnun costs about $15 and comes with screws, the one at Tower costs about $20-25 and does not come with screws. Don't ask me how I know but I do.