Hi grumpy, as usual, you have given good options there mate...I have some comments that may throw some more light on the matter....
Back in the 50's when this mower was first introduced, the first petrol engined Model 17's had a Villiers Mark 25C,147cc 2 stroke fitted. It was offered in 14", 16" 18" and 20" cutting widths. Later a 24" version was produced with a larger Villiers fitted and was a 4 stroke side valve...(The mark number escapes me).
Most of the 17's in service today have had Briggs or Honda's retrofitted as has this one....You can tell by the petrol tank and cowl in the picture...also the reduction gearbox on the engine.
Most machines also have a full length 'counter' shaft with pulleys (usually double) that can slide and lock in any position on that shaft as it runs the entire width of the mower and at the right-hand end has a dog for engaging a crank handle to start the engine (original configuration prior to recoil starters).
Engine mounting on these models is pretty versatile as is the horsepower rating used...I have seen 3.5 to 5 hp engines used successfully utilised.
