The round port heads on 1200's & A12's use 35 & 29mm valve sizes. The A14 uses 37 & 30mm valves.

You can port out an early head all you like & you still won't match the A14 head for port size right through to the valve.

Simply put, the factory matched each head well to the engine & its intended use.

If you use a small port head on the larger engines that came with an oval port head you will limit the upper end of the performance range.

By all means use a small port head to keep you mobile untill you can find a suitable replacement A14 head, or untill you can fix your old head, but I would never regard it as a permanant replacement unless the engine was intended as an industrial powerplant, or the vehicle was used in serious economy events.