If possible, I'd do a valve adjustment followed by dry and wet compression tests on your engine. If the results are positive, I would try to fix it. Start applying penetrating fluid (Kroil is best) to the broken stud twice daily. See what a machine shop would charge to drill it with a left hand drill bit. Often, that is enough to remove the stud. If not, use a square, not threaded, screw extractor, which you may be able to do yourself. Or have the machine shop do it. Also ask the machine shop what they would charge to drill the stud out and install a time-sert. Any qualified machine shop should be able to install a time-sert. Those studs get brittle with age, and if it were mine, I'd replace them all. Start giving each one a twice daily squirt of Kroil, and after 3-7 days, put two nuts on each stud (don't overtighten and snap the stud) and jam one nut against the other. Tap each end lightly with a hammer and then gently
try to tighten and then loosed each stud. You don't want to break them off and multiply your problems. If they don't loosen up, go back to the Kroil and tap/tighten/loosen until they do.
++ TIME-SERT Official Threaded inserts for stripped threads, blown out sparkplugs,
Used parts and engines are usually available on ebay as well as Jack's Rebel Warehouse Jack's Rebel Warehouse - Rebel Catalog, Honda Rebel parts and accessories
Jack doesn't list his inventory of used parts, so contact him. Low mileage engines on ebay run about $350-$400.
Unless the engine has been badly abused, 16,000 miles isn't much. Forum members have put over 100,000 miles on Rebel 250s. What gaskets need replacing, and why?
Here's a thread about the engines fitting.