Jimmy, here is the parts list page for the rear drum and its shaft:
[Linked Image]
[Linked Image]
I'm now less confident than I was that the sprocket simply screws onto a left hand thread and that is its only retention: there are some bits I can't account for. Can you post a close-up picture of the center of that drum sprocket, so we can try to understand how that bolt, no. 36, fits into the picture? It may be a complication.

A multipiece front roller has an advantage when turning the mower to follow a curved path, since the turn radius is different for different parts of the roller, and they should each rotate at slightly different speeds so they don't have to skid on the grass. This is the same principle that leads to the rear drum being in two pieces. I can't clearly recall whether ATCO used a multi-piece front roller like that, but I think they did. However Scott Bonnar 45 mowers have a single front roller, and seem to work satisfactorily.

A Briggs engine is a direct replacement for the HK25 except you need to check the crankshaft extension (PTO) length and diameter. There are both 5/8" and 3/4" diameter versions, and different lengths. However I do not know how the original Victa 2 stroke engine mounted: there has probably already been conversion work on that mower to fit the HK, so you may need to review that.