Aah, an A-series engine, a loong time since I worked on one of them.
Had a Mini 1275GT a very long time ago that required an engine rebuild, it was burning near a gallon of oil a month.
Found an Austin 1300GT in a scrappies with a good engine, knew it was essentially a 1275 Cooper S with the extra bolt/stud holding down head and the Cooper S hardened crank.
I stripped it completely and dropped off the block to be re-bored to 1320 I think, the crank/flywheel lightened and balanced, rods/pistons too at a local engineer known for engine work. I ported the head and combustion chambers with info lifted from my copy of David Vizard's book "How to Modify your Mini".
Didn't have twin SUs so carefully separated, well cut off the exhaust from the inlet manifold of the combined cast-iron inlet/exhaust manifold, then opened out and ported inlet manifold too. Another trip to a scrappies gained me an 1inch and 3/4 SU carb from a either Maxi or big 1800. A Minisport 731 cam and a Janspeed LCB exhaust manifold with a straight-thru box were also bought. Built all this and put it on the gearbox that came from 1300GT,
The big carb required a change of needle, this was a bit of a try it and see with more info this time gained from Clive Trickey's books which had needle charts and may have detailed which engines used them, can't remember but more needles were sourced from unknowing scrappies until I got the mixture correct, couldn't afford rolling road time or tuner to do it for me. Changed the springs in distributor to adjust advance curve and total advance, never quite got this right but was close enough.
No real idea of how much power it had but it must have been at least 90bhp going by it's performance, and acceleration was brutal aided by the 3.75 (or 3.65?) final drive in 1300GT gearbox instead of the original 1275GT 3.44.
I enjoyed driving this Mini, it had Spax adjustables, shortened cones to lower it, the brakes were the 1275GT discs on front, big Minifin drums on back.
Wheels were probably the original size, 12 inch 1275GT alloys or might have been 13" not sure. Would love to drive a similar Mini now to compare it to my present car an Audi A3 TDI S-Line with 170bhp. Think the Mini would disappoint me in my old age, like my comfort now.
Still miss the Minis, Hillman Imp with Sport engine, MK1 VW GTI, several VW Sciroccos with both 1600 carb and 1800inj engines. A BMW 325i with 170bhp, scary as my first powerul ish rearwheel drive car. Least said about my first ever diesel a Passat estate 1.9TDI bought mainly for the family to get us to Chamonix from Scotland loaded with camping gear for 4, climbing gear, 4 MTBs and roof-box, got 45mpg there and back.
Might not be a good idea to burst the good memory bubble of the 1275GT with the Cooper S engine though.
Edit to correct mph to mpg