I've got a 1946 morris 8 series E.
Back story is it was stored from 1966 until my dad got it around 8 yrs ago, then he got cancer soon after and I was determined to get it on the road before he died so he could have a run in it. No welding was required but lots of mechanical stuff was needed including a full engine rebuild. I did all this but it turned over very slowly and wouldn't start, starter handle attempts were fruitless, it was too tight. With the help of friends we tried a bump start and on the 3rd attempt she fired up and ran like a watch. Started instantly off the key after that. By then my dad was in a hospice and died before I could sort out the legal stuff and go for a run.
I lost interest for a few years and it's not been ru but now I'm back on it. Unfortunately it's doing the slow painful crank again and I've no where to bump it now, new battery(again), good cables and the starter wizzs over fine off the car.
Car is 6v but positive earth.
These starters are fine with 12v run through them but I don't want to convert the whole car to 12v so on to my question...
If I disconnect the heavy cable (neg) from the starter and connect a 12v battery via jump leads ,neg to starter terminal, pos to starter body , is that going to fry something? 12v pos earth running around circuits that are 6v pos earth?
Can't get me head around it.
I feel sure she will go with a decent crank speed.

You should be ok my old rally car had 24v starting to spin it over quick especially when hot ,here is a diag nicked of the net

With starter disconnected from original neg and replaced with your 12v supply shouldn't affect anything else. The positive will be common to the cars original electrics.
I assume when you connect the jump leads the starter will spin and there is no other connection to the starter.