Chris I would remove the main board and replace with good quality Digital Drives. Don't be tempted to try doing this on the cheap it will only cost you more money in the end.
I would also buy individual drives rather than All-in-0ne board like G540 or MX4660. Not because they are not good but rather because they are not cheap for what you get.

Steppers are most likely re-usable but you will need details to know how to set new drives etc. ie: Amps also how many wires to detemine how can be wired.

If power supply may be re-uasable depending on it's output. If less than 36Vdc then I'd replace it.

The rest will be re-usable, ie: E-stop, switches etc.

Now regards the BOB and parallel port then it's mine field.!! . . Cheap nasty Bob's can send you bonkers at best of time, couple these toPc with ropey parallel port and you'll soon be smashing your head against wall in frustration.
So my suggestion is Don't do it. Buy Decent Ethernet based Motion control card and connect it to Good BOB.

Only Motion control cards I suggest are ESS or Cslabs IP-M. Now when it comes to BOB's then my first choice is not to use one at all, which I'll tell how in min, but if I have to then only one I'll use is from PMDX in states but it's expensive. If really have to then I'll use one from Cnc4you or Roy at DIYCNC. Don't entertain cheap nast ones off ebay.

Now my first choice for machine like this and by long way is the IP-M. This removes all the BOB issues because doesn't use one and comes with all connection terminals. It uses 24v I/O so is very noise immune. Comes with Spindle speed Controller built into it.
It's very high quality unit which is robust and very neat. Manufacturer and software Support is very good.

Now at first it may seem expensive but if compraed to ESS and High Spec BOB with Spindle Speed control then it's not, it's actually much cheaper.

You could go with buying the G540 or Mx4660 and fit ESS but this will still workout more money than doing it with separate Drives and IP-M and still would have lesser quality system.