Had to do similar with a little fiat I used to race. A lot of overheating problems. Header tank was too low. Raising it higher than the engine cured the problem.

There's a vast amount of NGK spark plugs that are fakes . So good even motor factors as well as eBay have been fooled by them . Prone to failure, now I'm not saying it's the cause of your issue but if the centre electrode has let go it's possible that's done damage to bore. I've run Denso plugs for some years now and find them better in my motorcycles for what it's worth . I've never seen a centre break loose like that in all my years on spanners with both bikes and cars . With a 1293 tuned mini I once had running NGK plugs the bugger wouldn't start in cold weather, turned out be plugs , be fine all day once run once so since then always had a little thing about NGK plug heat coding . Replaced with champion plugs ( yeah it's a few years back Lol) and never had an issue with mini again . This is an informative reply from a 40 plus year dude on spanners , bikes and cars plus bike race mechanic for a few years , not an armchair diagnosis of failure . If you have batch of plugs bought at same time change them just in case . I retained some of the good old catalogue,s for spark plugs from the 1990's before we went digital and can cross reference plugs for you if needed . Ordering stuff today for vehicles I find older engine design using thinner spec oils , different grade of plugs etc yet the design not changed other than management systems and more sensors .

just make sure the header tank is as high as you can get it. ours was almost kissing the bonnet and if you want to cover your arse 2 ways my 1600m race car was fitted with an after market swirl pot as well hopefully the engine survived without bending your credit card to much.gary j [dave karaskas racing]

😢Good luck hope all goes well

Totally gutting, let's hope there's no lasting damage to that beautiful engine. I've never seen a 'spark plug' like that before...impressive! That needs to be put on a plinth or made into a a trophy for something! What ratio diff do you use, or is that a trade secret?

Something that i learnt in the past is that a high revving water pump (small pulley) can make a frothing of coolant in the system that creates air pockets.. Doesn`t seem to be related to this but i thought i would mention it. Are you using a standard radiator with the long filler neck?

LFG!

Can't you vacuum fill the coolant system?