I was hoping to avoid buying a new starter (The one I have is an inertia starter) and the light flywheel has the wrong type of ring gear for that starter. A stop gap measure would be to fit the heavy flywheel in the meantime until I can track down a pre-engaged starter.

The reason heat won't get the old ring gear off is when you heat it up it heats the cast iron flywheel too. The expansion rates are not much different so f-all happens.
If you got it all red hot it might come off but it'll be too soft to use, probably bent and the heat is likely to crack the flywheel too.

Like I said, new early starter ring gears are about A$50, at Kc and elsewhere. 3/8 or 1/2 wide, take your pick.

I cut old ones with an angle grinder almost through @ the bottom of a tooth, then hit it with a cold chisel and BFH. BANG and it's off.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Are there any inherent advantages to a pre-engaged starter? Otherwise I'll chase up a new starter ring when the proper (non-Repco/superhceap) shops open.

I was following the Haynes manual for installing the flywheel etc, but I'd already installed the drive straps and knocked over the lock tabs. So it was easier to make up the flywheel, friction plate and other bit on the bench, install it, put the flywheel bolt on, then install the pressure plate. Will this cause any problems with centering etc?

Some advantages of pre-engaged starter:
1. Don't chew the ring gear up
2. Don't flick out on engagement if motor gives a kick or 2 and dies
3. More starting torque (lots more if you go for Convertible Mini's Bosch geared one)
4. More durable, should last longer than the old Lucas inertia ones.

No problems the way you've installed it.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thanks again Doc. I've fitted the inertia starter and flywheel for now. I'll get it in the car and get it running, then chase up a Gemini starter.

Off to research Metro vs Mini cooling systems and wiring.