Looks like MTH rods would work with only a minor modification. Driver centers are the same at 4.5mm. Forward and rear bushing holes are the same at 4.46mm give or take a few 100ths. Center driver hole would have to be opened up about .020". That could be done with a tapered (aka mechanics) reamer. I am pretty sure MTH would have these as they have done dozens of J1s. Call or email Midge at MTH.
Not sure the cost but I would get two pair in case something went wrong. I would not count on Lionel to make them available even after their new J3s are released.
Pete