Consider getting a new short block for ~$2k from Jay Marks. It is by far the easiest and safest thing to do and may end up saving you some headache in the future. It is what I would do since your future power needs of the SP63 will be well within the limits of the stock bottom end.
Or, since Toyota only makes one size piston (stock size,)you can either buy six aftermarket ones the next size bigger and have the block bored, or have the #5 cylinder reseleved and buy one stock piston.
Since melting your #5 piston sent metal particals through out your block you must change rod and main bearing and oil pump and cooler anyway. If you go this route this needs to be done by a very good machince shop since Toyota offers 6 different rod bearings for the supra. I can not stress this last point enough.
Good luck,
Bill P