I agree with Tom 100%.In the old days(remember i'm old)the aussie formula 3 cars and clubman sports cars either ran a12's or the comparable toyota 1200cc power plant.If you did'nt have a minimum of 140-150 flywheel horses you would be best to leave the car on the trailer or be ready to cruise around at the tail of the field.
On the A10,those big numbers where possible but they where hand grenades and it was normally the 3 bearing crank that caused the dramas just as it is with the morris/MG products to this day

there was a bloke on the mini-list years back who raced a mini with a 970cc engine (large bore short stroke cooper S engine, much rarer than 1275 engines, same bore just way shorter stroke) and he got 121bhp per litre (easy to calculate since at the first oversize, the 970 is 999cc, or practically exactly one litre, so its power figure is also its power per litre.

Anyway he made 121 bhp and was spinning the thing to 11,000rpm.

The mini head, no matter how you port it is _nothing_ compared to a decent datto head, so the power figure is imho believeable. His valvesprings lasted something like 1-2 races tops (iirc) and he'd go through rods fairly often too - basically replacing them and lots of other bits at regular intervals before they let go and took the rest of the engine with them)

The cam he ran that gave power to 11,000 (i.e. he was upshifting at that mark or thereabouts) would, in highly tuned 1275s have them upshifting in the mid 8000s mark, a natural enough situation given they were much larger capacity, but similar bore, so they would struggle (relatively) to fill the cylinder on larger capacity engines through the same 'breathing' arrangement.

So it's imo, doable (not easy, but doable) but the rpm involved - even though it's shorter stroke, it's still 10,000rpm for the flywheel and valvetrain to deal with, not a small ask, and I would suspect valve springs would last perhaps 1-2 races tops.

Having a bit of a look at similar sizes motor bike engines, specifically the cbr1000 and cbr900, they have very similar strokes. In fact the 900 has a 58mm stroke, only 1mm away from the a10. This particular engine makes its power at 10,500rpm. Considering this, if you could run forged pistons and lightened balanced internals, could your reliability increase to beyond a couple of races?
EDIT: Is it just the new technology that is keeping these engines revolving so much faster, particularly in regards to the valve train?

Its been a while since I started this thread and from what I gather
if an a12 can make 150hp with an oval port head an a10 with the
same head could achieve 130hp at or higher rpm. Im sure Gordon Dobey
said he rebuilds the a10 every half a dozen race meets and chances
the oil after every meet. Theoretically an A10 with strengthened balanced
internals good flowing head, well setup rocker gear, 59mm stroke and good
easy rod stroke ratio shouldnt spin that high and make those numbers.

Yet since seen TopGear's 98kwatw A12...I prefer an A10 with 85kwatw @ 5500rpm
anyday with cheap well setup forced induction.