I have done work arounds as well. Small mod to the battery case on Metz 60 series flash generator, to allow me to use a gel cell meant for a UPS etc instead of the OEM dryfit. $20 vs $65.
Braun flash from the 60's - there is no longer a vendor who make 4V wet cells. I needed 8V; I did find 2V spiral wound gel cells, and so now 4 of them live in a sub case that fits into the bottom of an 'overcase' that the original flash generator sits in.
Take care with the NiMH is you are not replacing the charger, and it was NiCad. The Nicads had a need for slow charge rates. If you exhaust the NiMH, it could be days before theya are fully charged on a NiCad charger.