Hi Steve
The b2 piston seems to work well enough and your Vortek adaptor and seal should fit if it has a 5mm retaining bolt.
The b2 piston could be spun by the latchrod in an electric drill to linish the bearing surfaces down to fit the 25mm Weihrauch tube.
I increased the depth of the cutout in the latchrod using a 4" angle grinder with a very small cutting disc in it, sounds bad but I use angle grinders on a daily basis so I didn't even worry about it --- a Dremel should do the same job, but slower.
The HW50 I've used as a guinea pig is running at 8ftlbs in .22" and has a really nice firing cycle, in fact it's that nice I may just keep it at 8ftlbs as a close range ratting gun.