What is the right A/C contactor part that I need?
I searched R8243 and came up with a Honeywell cross R4243b1012.
Part numbers change all the time but this should be a compatible part 24v coil and a double pole contact set without knowing your compressor size it would be hard to guess at the right size because Honeywell did not provide the specs for the original.
It’s not a big deal as long as the control voltage 24v is the same   It’s a double pole motor contactor the last part would be the HP rating.  Well and it fits. It won’t hurt to get a larger one as long as it fits. I put larger ones in all the time the contacts actually last longer.