Hello Claire,
The more common later Mercedes five speed manual transmission will bolt-up nicely to a W113 engine. The biggest engineering challenge will be hooking up the shift linkage since these units are three rod with a side shift cover. The original set-up is a single shift rod and top shift rod cover. The latter unit also has a different driveshaft flange. A new driveshaft front half can easily be made or modified to work by a driveshaft shop. A shifter for the latter transmission can be used from a donor car. The W114, W115, and some W123 Mercedes used this later 5 speed as an option. Around $500.00 should get you a good used one. Keep us up to date!
Joe Alexander
Blacklick, Ohio