I emailed the inverter drive supermarket and the guy called me back almost instantly. We chatted for quite some time, he didn't mind wasting his time with me, a very nice bloke.
It seems there are already sliding plate adaptors for this job, which he emailed me the spec sheet for. What I need to do now is remove the old motor and take measurements, then get back to him and he'll sort me out with the motor and plate that I need. I may even be able to get a larger cage motor in there, if I fancy going up to 1.5hp.
He also explained to me that you don't lose power in the lower rev range, only when you go higher than the specified speed of the motor do you lose a bit of power.
I have to be careful where I mount the VFD, it's susceptible to dust, swarf, damp etc. I'll also have to figure out what I want in the way of switching but that will all come when I've sorted the motor.
All in all, a fruitful conversation, can't recommend that company enough.