In UK Velorusion / Bike Republic stock remaining Mk1 strida spares (Not all parts are available … but I think most of the plastic parts are).
velorution.com/
bikerepublicstore.co.uk/
Otherwise as previously mentioned: Ebay UK.
OR … as the Mk2 is same 100Teeth (but grey) that would fit.
And with some ‘adaption’ (to either axle ie converting from BMX style bearings to normal BB )
or adapting front freewheel to match a Mk5 100T Front pulley (or using whats left of Mk1 front pulley as an adapter to a Mk5 f pulley).
Finally … how broken is the Mk1 F pulley ? … cracks can be fixed, and the flange rim is not necessary for drive … maybe it can be repaired (if a bit ugly 
)
Tom_T