Hi Andy.
I am really wonder with your project.
May be It would be a new chance for everybody that owns a Strida Sport Duo instead of fitting large wheels, because the mudguards dont match, the big tyres are less comfortable, more expensive and less available. As the fitting 18" wheels purposse in the SD is rising the speed, may be, we could get the same speed in our duos with a different alternative.
The alternative would be to fit the original plastic Strida front pulley to the current Speed drive and keep the 16" wheels. Unfortunattely, the original Strida 5 plastic pulley has four inner spokes instead of the five ones the miny pulley has. One solution would be to use the not holed Strida 3 plastic pulley, although I dont know if , after chopping, the structural strengh of this pulley would be enough.
At last, We could use the new alloy strida 5 pulley (optional), chopping it and adapting an intermediate thin alloy plate.
Regards
Hydra