That is an issue. It doesn't sound like compatibility would be easy or cost-effective.
Would the bellset/bell from a junked GPO phone work with the U43? These seem easy to come by.
The diodes are instead of a No 205 rectifier as suggested by
https://www.britishtelephones.com/tranchan.htm
Phones are entirely new to me, and I didn't want to blast my families hearing, so I went with the instruction.