The 1974 Johnson Parts reference Guide, which lists what parts can be substituted with other models, does not show that as a possible swap. Note that the year after your RD-18, 30 HP was made; the RD-19, 35 HP in 1957, got upgrades in the gearcase to make it hold up better to the extra 5 HP. Also, the guide lists the RX-13 and up gearcase assemblies compatible with each other, but not with RX-12 and lower. Not sure why, except the RXs through RX-12 were 28 HP and the RX-13 and up were 33 HP. I have found the guide is not always totally correct on things like this, so – maybe? Someone else here may know for sure. If nobody knows for sure, I would pull the gearcases off both motors and check for for differences in driveshaft dimensions and splines, and dimension of where the water pump drive pin is. — maybe try putting the RD-18 driveshaft into the RX-13’s lower and see if it allows one or the other of the water pumps to be installed, and check/compare driveshaft length above the gearcase mounting surface to see if it has changed. Also check how far the shift rod sticks up. If one of your motors is short shaft and the other a long shaft, it maybe possible to swap parts between them to convert, but to swap the shift rod, you have to disassemble the gearcase.
Dave
