Receive my kit today !
I just take some measurements...
Post corrected with new measurements
Max distance center of spur shaft to center of motor shaft = 28.5mm
Min distance center of spur shaft to center of motor shaft = 24.5mm
Max pinion size with plastic spacer = 18.3mm or a 43T 64P pinion
Max pinion size with just the brass sleeve = 20.9mm or a 49T 64P pinion
The lowest FDR I was able to achieve with my spare parts is 3.88 with an 76T spur and a 49 pinion !
I don't think it's possible to have a lower FDR without a different motor mount...
Your millage may vary with different spur/pinion manufacturer.
I will try to dremel the motor spacer to have just the sleeve as the limiting factor...a picture may follow this weekend if somebody is not faster than me
In my opinion this kit is definitively usable with a 17.5 and not optimal for higher turn without a new motor mount redesign !