Specifically, if you still do have the original Denford-fitted VFD, then this is a pointer to the manual for it (pointer to PDF in second post).

The Parajust VFD was a fairly early VFD, and from what I have read does not have the greatest reputation for reliability. However, when working, it does do the job. If you are going to use it, read the manual very carefully about connecting to it as some of the input terminals might carry dangerous voltages. Certainly you would be OK switching the forward/reverse inputs via a relay which would provide isolation, but I'm not clear about the 0-10V control voltage input. The manual describes how to use this, but as I say, I'm not sure about isolation from high voltages. In any case, you are going to need a PWM to analogue voltage converter, the same way that I did. The P-CNC board has a PWM output, not analogue, which seems a bit silly as almost everything wants analogue. However, you can pick one up for around a fiver off eBay. I used one of these but there are plenty like it. I didn't need to look for high-voltage isolation so didn't bother to check if these cheap little boards would do the job in your case.

If there is any doubt about the Parajust VFD (if that is what is in there), then replacing it with a modern equivalent is very easy.

Sorry for slow reply - the site seemed to be unavailable for a couple of days.