"Its the two mounting studs into the stringers that are spinning and can't remove them"
And the stringers are wood or wood encased in fiberglass?
It could be the wood is badly rotted out, in which case you'd want to rebuild the stringers. Basically, cut them out, lay in new 2x4's or whatever, glass them over.
It could also just be that the bolts worked loose or the wood rotted just locally around them. In either of those cases, you should be able to drill a couple of 1/4" or so holes, clean out the area as best you can, and pour in epoxy to embed the bolts good as new.
Rubber part makers in general say that all rubber parts--including engine mounts--should be replaced every 5 years, as the rubber stiffens or breaks down. So new engine mounts are called for in any case.
Fortunately you're already planning to pull the engine, making the engine mounts a much easier job. (Looking on the brightside.<G>)