Yeah, I think I will go with a Titan, I don't like how tight the coils are on the OX & I suspect it's going to be harsh. I put a Titan in my Hatsan & it's got a very pleasant soft cycle, I did need to take quite a few coils off because its .25 . The only difference is that's pushing a big heavy piston in the Hatty & the Originals piston is small & quite light.
My mk1 HW 97 had what I thought was an Ox in it, but putting it next to a real Ox I noticed that it's coils go the other way around, then someone told me that Weihrauch used square section springs at one point, which seems odd if it is the standard spring because it was almost coil bound when cocked.
Good luck with your mums op timing & I hope it all goes very smoothly for her, Mike.
Edit, Titan XS number 3 ordered
.