Hey guys just looking at racing classes and was wondering if u could make a a12 to under 1100cc's? I know there's an a10 but its only a 3 bearing crank

If you sleeve the bore to 70mm
Would make it a 'square' engine then
Good in theory

Or you could have a competant machine shop destroke the crankshaft, but then you would run into all sorts of issues like conrod length and piston pin height.
Either way it could get real expensive.

Remove 1 spark plug.

I guess you must be thinking about Historic GroupNc up to 1100cc? You can't do it under the rules. You's have to run standard stroke, and can only overbore it by 1.5mm (.060"). There is no freedom to reduce bore size. The intent of Historic racing is for cars to run as they were in the era,a nd you have to comply with the specification sheet for your model.

Harry are you allowed a different head other than Nissan or stock?

If so a stock Mazda 1300 16v head with 40mm dellortos, 2 exhaust cams,
pipes & megajolt should make nice hp at 10,000rpm if A10 bottom end done right.

Dont know how reliable that figure was but a few of Gordon historic 1000
A10 engines supposedly made 130hp ate with A series heads.
His A10 engines ran prepped rods, Arp rod bolts, full floating forgies with
ladder brace from laser cut 8mm steel and cheaply sourced unbreako main bolts

Mazda heads flow alot more an extra 30-40hp should be possible.

Not for historic but yes for I.P.