was the old piston a cast piston?...if yes, you won't be able to just throw a Wossner in it, because they're forged, and require a greater tolerance over a cast piston...and it'll seize up tight once the piston gets hot and expands...probably less than 5 minutes.
At this point, you'll either need to go with another cast piston...I think Pro-X makes over-sized cast pistons in 1/2mm increments(if needed)......or, send the cylinder out to be re-plated, and fitted to the correct tolerance for a forged Wossner.
Do you still have the old piston?
You need to find out what the cylinder bore measures right now, before going any further.
If it were me, I would send out the cylinder, and have it re-plated, and set up for a Wossner...that way you'll know it's all set, and good to go.
I know you're on a budget, and probably didn't want to hear any of this...so you'll have to decide which way you want to go with it.