The elec tach won't work without being internally re-wired. It's not as easy as just changing the direction of the loop. If it is the loop type, then one possibility would be to use a DC-DC converter to supply it with the -12 volts (relative to ground) that it wants. Haven't tried it myself, but should work.
MGA definitely used a "balanced" fuel gauge, no VS required. An electronic VS won't hurt (the gauge will still work on a steady 10v) but an original type VS will keep it from working properly. Notice that the gauge housing (assuming you have an original MGA gauge) needs to be grounded for proper operation. Without the ground, they will typically read either past full, or past empty.
It's been too long, I don't recall for certain, but I think I remember a buddy's MGA being wired the same as a TR3, which is to say they relied on the gauge panel to ground the gauge. If so, I suggest adding a ground wire for it.
With the balanced gauges, an open circuit to the tank sender (or a broken wire inside the sender) will cause the gauge to read past full.