You need some kind of collar made from pipe that fits nicely over the axle stub and a drill size to match the outer size of the pipe.
Maybe try wheels from another brand of mower. They seem to come in standard wheel circumference sizes in all the brands of six, seven and eight inches and all seem to have the same axle thickness besides Victa with their slightly thicker axle stubs.
The width of the wheels could vary a lot though!

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!