Hi Yves,
You will need to buy 4 identical gears, of a smaller size than the ones currently installed (so that the 2 gears which get installed to the shafts do not touch each other). If you look on ebay, I am sure you will find many.
I will email you some pictures I have of the original Maudslay mechanism.
I think PB stands for Peter Baylis, the maker of the engines and 2 for the number of cylinders. There were also PB1, PB3 and PB4 engines.
Besides, there was also an even earlier version of the PB2 than yours, which had the steam chest arranged longitudinally, at the opposite ends of the engine. It had a similar slip eccentric with o-ring reversing mechanism, but arranged differently.
And, as you know, there was the later version of the PB2 with Maudslay gear.