but why spend $2500 on a plough base with an engine on top
That's poetry to me, Kye ...

There was a lot of discussion about the Victa airfilter that is now over $200 on eBay
Folks must have more money than sense.
For me, this is an ancillary part and does not go to the essence
of a vintage mower. I would much prefer a new, quality reproduction
part on a resto. I hope 3D printing develops to the stage where these
parts will become available at sensible prices, and will be
indistinguishable from the original.
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Jack