A14 head won't lower compression. Most A14 heads are the same CCs as A12, although some later A12 & A14 heads (certain 1982 and newer) have a slightly larger chamber. See Chamber Size. In A12 engines, there are many piston dish sizes, which is primarily what determines compression ratio

All A14 heads come with the larger valves. All A12 heads have the smaller valves. By the time you add cost of new valves, and machine shop costs, it's usually better to obtain a good used A14 head. It's factory ported for larger flow already (80 HP vs 69 hp). Porting the A12 head makes sense when racing class rules don't allow head swaps, or when you are doing all the labor yourself