Yeh BT45 if u can find them in 16's however they will be slightly taller as the gpx twofiddy runs 80 profile and you'll fund the BT45 is 90 which might mean some front guard rubbing..
Check out Pirelli MT75 for some cheap but still good tyres that will match the oem sizes of 100/80-16 front and 120/80-16 rear have a browse around some local shops n see what they have
I found the biggest range of 16" tyres was at a local scooter shop since some of the bigger scoots these days runs 16" tyres which match up pretty well to the 80s bikes of which the gpx comes from
just remember whichever tyres u get will be very greasy n slippery on the road for the first 50km or so so don't tear off down the road straight after they are fitted!
