Do a simple compression test on all 6 cylinders, that will give you a much better idea if you need a total rebuild or just a valve job. If the other 5 cylinders are all within 10% of each other and not too low compared to the factory spec (be sure and adjust for the elevation difference:actual reading divided by .86) you can probably get by without a full rebuild. 0 compression on 1 cylinder could be a blown head gasket, a burnt valce or a cracked valve seat. All are fixed by pulling the head as the first step.
If the engine isn't burning oil in addition to the #2 cylinder issue you probably should just do the valve job.
I am not a big fan of installing used engines unless you are totally desperate or unless you know the engine's real history. Otherwise you could go to a lot of effort to install something that could start giving you a lot of trouble right away.
Randy Rosetta KDØKWX TLCA #19351 69 FJ40 "Meanie" & 1970 Boss 302 Mustang