Having just rebuilt an RC54-G20 deck, I am familiar with your dilemma. You should probably remove the belt and check the spindle bearings for excessive side play, if they haven't been getting grease regularly. Were they making a lot of noise?
The 'tube' that holds the grease zerk, looks like a typical 1/4" pipe nipple (probably metric), but with internal threads at the top end for the zerk to screw into. The lower end of that nipple screws into an ear off the CAST IRON spindle housing/bearing holder and the top end just protrudes through the deck shell. It might be difficult to dig out the remnants of the broken tube or rehab the female threads in the cast iron bearing holder...
If you have to do that, you're going to be removing the spindle assembly to get access to it.
Here's a detail pic:
View attachment 88082
Nevermind that I have the wrong zerk installed in this pic. It should be the straight one, not the angled one, so that it is accessible through the corresponding hole in the cover.
Here's a detail pic from the Parts List:
View attachment 88083
Not sure why they made the RH spindle different than the LH. LH is handier to grease because the zerk is screwed right into the end of the shaft, which is bored to deliver the grease to the bearings.
I'd be very tempted to get another LH spindle assembly and install that on the R side. Really, you just need the spindle itself. PN of that LH spindle is 76539-34550 if your deck's serial number is >=20001.
PN for the "Pipe Extension" (Part 180 in the diagram) that holds the zerk is 76539-34132. Part 190 in the diagram is a set screw, PN 03410-50606.
The grease is delivered to the same place on both RH and LH spindles - between the upper and lower bearings.
Good Luck,
-Paul