Thanks for the tips AZL57. For testing the engine I'm planning to go fully 12V. I'll use a spare 12V coil and possibly run the D*lly or my 12V bike as a generator. Only the ignition and starter will be connected, the engine is nowhere near the car (20 miles or so!).
I have a few starter motors lying around but the 6V one really isn't looking too hot. A 12V one shows signs of life and I have dismantled it to attempt resuscitation. It may be that I have to borrow the D*lly's.

You only need a battery, not generator. 6v starter motor will take 12v, certainly in the short term - spins the engine faster too

Ha, well that depends how long it is before I get signs of life: tinker, spin engine, scratch head, cup of tea, repeat!
I don't fancy just using the handle so at the moment it's a case of any starter motor will do, use a small 12V battery I have with a running engine keeping the Vs up.
Next up is investigating why a pool of oil has appeared under one of the heads