Yes, I agree it is nice to keep the rare ones original, but to what end?
The cost of refurbishing another engine and drive (for the twin) alone is more than the difference in resale. Then there is extra running and maintainance costs over the life of the boat. So, on a dollar for dollar rationale it doesn't pan out (not even close).
I have seen many classic boats, some valued in the 100's of thousands of dollars that have had extensive refits including non original power. It has not greatly detracted from their value and has actually made them more enjoyable to use (the original purpose). As long as it is documented it's all good. 40 years from now if the boat is actually worth it, someone can refigure it to original specs.
The issue of "the last few standing" is romantic/emotional or historic, or maybe has somthing to do with brand loyalty but, other than bragging rights to a rare model, it does nothing. We can all say "it's a nice boat" though.
On the practical side, you must consider the utility derrived from this boat. The owner bought it for a reason. I'm sure the refit of this boat helps the desired application of its use. To maintain a configuration which is not conducive to it's use does not make sense. Makes less sense if maintaining that configuration actually detracts from the intended use or increases costs for the intended use.
Now before you think I'm a totally heinous person without respect for provinance, (I've been through alot of this with the Porsche Club of America..). I would not for example, repower an original 1973 911RS 2.7 Carrera with a 3.2 L motor...but it's inherant value reflects this.
Similarly, If this GT 21 twin is such a rare and valuable boat then theoretically someone should step up and offer a very large sum of cash to buy it (and keep it original). As it's only worth what someone will pay, I fear keeping it original, while nice a sentiment just isn't practical.
Bottom line is...it's up to the owner. He should make it how he wants it and enjoy...no matter which configuration he chooses. That way he can't lose.
Sean Conroy,
1964 Formula Jr. (hull #2) project
1972 Greavette Sunflash III
1981 Kavalk Mistral project
"A man can accomplish anything... as long as he doesn't care who gets the credit."