The reason your dealer is being less than helpful is there is not an easy to find combination that works on that model.
There was no OEM R4 size tire and wheel combo that fits it.
I'm not able to find an R4 or even an R14 that will fit either your existing front or rear rims.
You would need to find out the RC ( rolling circumference) of the front and the rear tires then get that ratio.
Find a set of rims and tires that will fit your tractor in that ratio.
I would like to be able to tell you you could just get a set of tires and rims off of a LXXXX but there is not charts or documentation on doing that, or at least none that I know of.