Tyres:
Bridgestone BT45s seem to be the popular choice for ER-5 owners, being rated as "Very good".
Exhaust:
A lot of bikes run with the original system, although one respondee had fitted a Remus system "Noisy, but nice!"
Rear suspension:
All bikes in our survey were fitted with OE shocks.
Other mods:
Crash bars and screens are popular accessories