Old RWI B12 robot parts

I have an RWI B12 robot that has been faithfully working for me :-) over the years (since 1995). Unfortunately, his plastic (?) tires are beginning to crack and split. Epoxy will
hold repair them for a short period but new cracks are appearing to the point that soon the tires will be more epoxy than tire :-(
Anybody been in a similar situation and have insights on a decent replacement tire or tire/wheel combo? I'd rather leave as much of the original equipment there as possible, so in an ideal world I would like to just replace the tires and leave the wheels intact.
thanks, kenny
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