"It might be possible someday using brain scan to know for sure what other (scientific or whatever) fields Bobby Fischer's mind could have been suited for?"
Okay I see that might not be possible directly but it could still be possible indirectly. Suppose in the future, brain scans are so accurate that it's possible to know what type of minds from the past belongs to what (pretend) category. Pretend the following is true:
Brain category: mathematical
Subcategory: chess players
physicists
inventors
cipher breakers
computer programmers
musicians
So, by knowing that Bobby Fischer was a proven great chess player, and since physicist is in the same (pretend) subcategory as chess player, there would be no need to actually have Bobby Fischer's physical mind for testing.
Bollocks?