So, a couple of thoughts here.
First - the fecudity of such a older fish is questionable - meaning that its ability to generate a good quantity of eggs is inferior to a much smaller fish. Not to say it would be sterile, but the eggs would probably be very diminished in both quantity and quality. At least that's what I read in the past.
Second - the odds of such a large fish surviving what must have been a titanic fight are probably not great, particularly so given its advanced age, and also would greatly depend on how it was handled once landed. I can't really say, but do suspect that a smaller, less age-ed fish (say, in the 10 to 15lb-class) would have a better recovery rate from such an event, taken in the whole.
Third - as to a taxidermist having a mold that would properly emulate the monstrous size of such a blackfish - yeah, also doubtful. How many time would any one taxidermist have seen a 20lb+ tog, that he would have been able to record the relavent dimensions - and then create (or order) such a mold? Probably nil on this.
I for one would greatly prefer a gentle return of such a fish to the water - depending on its condition. If it looked like it would swim off, then fine, off it goes. If its bleeding heavily, or just can't seem to regain its strength, then most probably its would be heading to a good taxi-man, for a proper old school "skin mount."