I managed to obtain some Heljan Hymek spare part sprues, which include the bogie gear casings:
This close-up shows the seven fixed pegs on which the worm wheel and idler spur gears revolve:
On the face of it, it looks like a new central axle can be run straight through, in the exact middle of the 10'6" wheelbase. Heljan, of course, don't provide any cogs as separate parts, so that makes the job of working out clearances just a little more difficult.