Hi exterminator and welcome to the forum. Those Princess 32's were THE boats to have when they metamorphosised from the Project 32. The twin engine versions would almost plane and the singles made economical river boats.
That doesn't help you though. When I first looked at the photos I immediately thought oh that's a Perkins engine, it's certainly a Perkins colour scheme, but of course it may have been repainted. Volvo engines are normally a little darker green.
My memory is not as good as it used to be but what I can tell you is that Perkins and Volvo used the same block which I am pretty certain is a Peugeot engine marinised. The XD 4 marking was something to do with the type of car engine which I'm certain was used in a Peugeot 504.
Now this doesn't help much with the original question of identifying the type i.e. is it an MD21 or an AQD21 but it may help with finding a lift pump for a Peugeot somewhere which will do the job.

Edit. It's a Peugeot - Indenor XD P 4/90 Type 4 I think marinised by Volvo or Perkins