You will have a lot less grief, & save yourself some cash if you start with an A15 head in the first place as it will have the oval ports already & come with the 30mm exhaust & 37mm inlet valves as standard.
You may need to shave the head a little to restore compression, & if it is going onto a 1200 engine, you will need to cary out a small mod to ensure that oil gets to the rockers as this part differed between the earlier & later engines.

You will not successfully port a round-port head out to the oval-port size & shape, at least, not cheaply.