The mountings for all VS800 engines are the same, so virtually any VS800 Intruder or S50 Boulevard engine will fit. What changed over the years is the electronic ignitions and details of the carburetors. Also, California models have extra parts you won't want (unless yours IS a CA model).
If you find a used engine in good condition, try to get the matching ignition electronics, at the very least. Your carbs will work just fine, if the used engine has none.
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'68-7? CB450 and '98 Valkyrie Tourer, too