If the cambelt hasn't been changed then defo get it done mate as these have been known to fail at 60k.
Bit difficult to gauge power in comparison to a 130 Mondy as even I think a Mondy should feel like it pulls better through the gears although the ST170 is quite rev happy. IMRC faults are common on these and sometimes do not bring up an EML so check the IMRC unit and the runner control to see if it is stuck open. Other than that, when was it last serviced? At that mileage if you renew the coil leads and also a fresh set of plugs, it may give you back some lost performance from over the years. Similarly with the fuel filter if it hasn't been changed in yonks then I'd advise to do so.
Only other thing is that with the ST170, the engine output was literally pot luck as the quality control when machining the heads wasn't very tight so it could well be you unfortunately have one of the lower power ones. Some made as low as 155bhp. Typically they are all around 160-170bhp.