Well, in the absence of replies from readers that have experience with this sort of mods on their receivers, I did a little searching on my own.
It seems the 'ole standby FET for this vintage receiver as well as many others, was the 2SK125. It has been discontinued, but I'm sure it can still be purchased.
Looking at my receiver, the FET's are in a balanced mixer configuration as the "front end". There is no RF preamp section ahead of the mixer... only bandpass filtering.
So... it seems that the J310 FET is the device of choice for updating the 2SK125's. The two 2SK125 FET's are on the underside of the main board, in a SOT-23 package... not much bigger in size than a surface mount chip cap or resistor.
It's hard to determine if there will be much in the way of improvement in performance. I would assume the main "gain" would be in the ability to handle larger sigs without "overload". I tried to compare noise figures but the specs used different freqs for their noise figures. I would hope to see a bit better performance on the 10-11 meter bands, which is what I'm looking for.
I understand that any "gains" might only be seen on a test bench, and not by casual tuning through the bands. The only "negative" I see is that I may have to rebalance the mixer after the swap-out. The devices are pretty cheap, so cost isn't a drawback.
I've never been one to "leave well enough alone" (or if it ain't broke don't fix it).
I'll most likely give this a shot in a few weeks. Before I do, I should try and document some signal levels from LW to 10 meters. Unfortunately, I don't have the equipment to do a real check on stats such as image rejection, sensitivity above 11 meters, etc.
Until then...