Pete
Might I ask why you want to convert a radio which you can use for a submarine into one which you can't? Digifleet radio was one of the best and most sought-after sets of its time, but manufacture ceased well before the advent of 2G4 sets so I doubt you would be able to find a conversion module which would just drop in. You'd also be left with a redundant receiver, and servos which would need modification before they could be used with a contemporary 2G4 receiver.
My advice would be either to keep it for one of your subs, or sell it (I think it would be snapped up quickly) and buy a new 2G4 set with the proceeds.
Suit yourself, m'duck.
Dave M