Wheels / tyres for Ransomes Multimower (3.50-7)

I'm finding it difficult to source tyres or tyres + wheels for a Ransomes Multimower. I have a complete gearbox assembly with two wheels (the latter are in poor but usuable condition) that I intend to repurpose as a garden tractor unit. I'm therefore not fussy about keeping to the original size and slightly more ground clearance might even be useful. The perished tyres that came with the gearbox were 3.50-7

7" appears to be an unusual size. 8" is much easier to find.

Would anyone happen to know of a suitable source of tyres and/or wheels that would simplify this build? Otherwise I may have to fabricate an adapter plate to bolt to the wheel hub (p/n MBA1072 found on page 18).

Many thanks,

Brian