Hi. Would like to share with you it is possible to change the tyres without new homolgation, at least here at Spain: trail ones that seems to be fine ( 12 km only) at non asphalted roads.
Pirelli SCORPION RALLY STR 110/70 R 17 M/C 54H M+S TL front
Pirelli SCORPION RALLY STR 120/70 R 17 M/C 58H M+S TL rear
Do you feel like you'd be able to do 120/80 on the rear without getting rid of the fender? These look great tho as is. I'll be looking more for street tires for my commute, but I always enjoy this look.
Use a tire calculator, compare size to original dimension and see if it room for some extra millimeters.
I changed to 100/90-17 on my TS (from 100/70-17) 20mm higher, had to lift rear fender a few mm, doubt 120/80 will fit, maybe 120/70 but it is not much room for a wider tire.
Check that you have at least 10mm between the screen and the deck everywhere (plus a few mm clearance)
This guy on FB has changed to 120/70-17 on TC Max? (belt drive hanging down) Dont know how different they are.