You mentioned that you thought that only a couple of threads were messed up, but you don't cite on which part. I assume that the 'messed up' threads are on the cyl head. Is this correct?

If so, then can you thread the plug with it's thread insert on it into the hole without cross-treading it?

If the answer to this is YES, then you may be able to make it work without removing the cyl head.

If you can thread it in, then you can evaluate by feel the sizing of the pitch diameter (PD) of the thread insert sized internal thread in the plug hole. If the PD's of the internal & external thread get too much allowance (gap), then you will feel a lot of wiggle when the two threads are engaged but not locked down. This is the aspect to evaluate if you can get that insert, on the plug, threaded into the hole.

If the wiggle is not too bad, then I think you could make a reliable joint by threading the cleaned insert into the hole with some JB Weld on the threads. I know this may sound rather drastic, but if you're at the point of taking off the head or getting rid of the car, then it would seem an option worth considering.

It's viability is hinged on you being able to get the insert threaded in and not have a lot of wiggle in the fit.

If your evaluation meets these criteria and you decide to do it, then I would suggest removing the insert from the plug and cleaning it well with Brakekleen, alcohol or some non-greasy solvent. Then put a little anti-sieze on the plug external threads. Then thread the cleaned insert on to the plug threads IN/OUT and look to make sure you've got a good anti-sieze coverage while still making sure you've kept the insert external threads clean and snug the insert onto the plug as best as possible with pliers & a plug wrench or spanner. Then tooth-brush clean the plug hole with alcohol or the like.

Carefully apply a little JB Weld to the entrance of the plug hole and try to get some coverage around the full circle.

Apply the JB Weld to the external threads on the insert on the plug, but don't go all the way to the leading edge, but stay back 1-2 threads. This will keep the JB weld from getting in front of the threads. Use a brush gently to get even coverage on the rest of the threads behind those lead threads.

Thread the insert in on the plug with maybe a little back & forth like using a cutting tap; in a couple turns, back a quarter turn, in another couple of turns, back a quarter, etc.

Once you get to the bottom, be very gentle in tightening. Use a plug socket feeling the torque very carefully; you don't want the aluminum to yield. Even about 5 ft-lb torque would be acceptable as the JB Weld will greatly enhance the load bearing capacity of the threaded joint where the allowance in the fit of the PD's is excessive.

Let the JB Weld cure overnight, remove the plug, inspect everything, re-thread the plug in with anti-sieze to ~15 ft-lbs and see if it holds.

I think it might be a viable option.