Can anyone tell me how much more power an oval port head will generate on a 1200 please

5 HP

If you are not revving it up to 6000 rpm, 0 HP.

i will be reving the engine... its in an Austin Healey Sprite race car... so not a lot of difference then maybe 5% ??

It does make a little bit more power.. but its moer of the availability of using twin carbs etc that go better this that head and manifold etc, that are more readily available. So from both a parts and power view the oval port trumps the round port.

The head will make little difference without a camshaft change and carburetor change. Then it would be good for 80 HP at 6000 rpm up from 69 HP.

Im running a 40 weber Dcoe and 37/70 cam

Oval port H89 heads have way larger inlet and exhaust valves from factory
ports larger but smaller than oversized GX yet just right for all round
performance much like the big port FIA heads that where made later with
smaller ports as it improves the midrange torque.
The oval ports also have higher port position and really are great addition
to a bigger A series or high performance engine.
To get a round port to flow close to an Oval port head theres is quite alot
of wallet clearing and custom manifolding not designed for big round ports.