Phil, I have no experience with lightened flywheels though might add to the list of those who suggest to leave it standard as many a tuning effort ends up a disappointment as the initial balance of the engine can be upset.
I do have some experience of replacing a core plug on that block, though in a Lotus Elan and at the side of the road, it was a long time ago. I used a coin of the realm, I had a ball peen hammer in my tool kit and dished the coin a little bit, also used a smear of Araldite round the edge of the coin, which if I remember correctly was a 2p.. (-: Put the coin in the block with the now raised side facing out and tapped it to make it expand into the block for to tighten the fit.... worked a treat.. (-:
It's very frustrating that of the 4 plugs, it's the one that's most difficult to get to! Reckon it's going to be several hundreds to fix for a 50p part...
