That’s good news with finding the Westwood orange paint.
Regarding the engine, I would recommend finding an engine that is as near a fit as the original was. This means getting a suitably powerful enough engine that also has the correct shaft length and diameter that the original drive pulley size will fit onto.
I’d certainly make sure that pulley ratios are correct and that the drive belts are still able to follow their proper routes once the engine is in place.
It is more than possible to fit a different engine than the original specification, but over the years we have had fun getting some alternative stuff to fit!
As a funny story of what can happen, several years ago I had an Atco ride-on mower (minus deck) with 8hp Briggs engine which died. This necessitated a replacement of which I handily had a 6hp Robin engine from a Gravely push mower, and after many hours of tinkering and finding the only drive pulley that’d fit the drive shaft, but was much larger overall than the original, all seemed good. I cannot recall the exact specs of the Robin engine apart from the fact it’s performance was greater then the outgoing Briggs, it’s performance was such that we fitted my bike speedometer and it achieved well over 20mph at full speed which is a terrifying speed on a ride-on over a grassy field especially when one hits some lumpy ground. Engine power, RPM, and pulley size are really important to safety!
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