This engine is scrap.

I suggest you buy a good, running, used 2.1 or 1.9 (I think Ken Wilford
had one recently) and put it in.  Replace the heads if you want with
your rebuilt ones.  Take your now "core" engine and sell it to a salvage
yard or rebuilder for $250-$300.  If one stud was that badly corroded
the others can't be much better.

Did the stud break at the case end?  If so, only a machine shop will be
able to drill the end out and repair the threads, and the case will need
to be spilt.  Have them replace the others too if you opt to fix this
engine, but I really don't think it is worth it.
--
Stuart MacMillan
Manager, Case Program
800-909-8244 ext 208

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http://www.casedealer.com/demo
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