The G5 plug indeed lets one get away with a "quick&dirty" conversion without the sparkybox and associated adjustments etc etc. But it surrenders you to difficult to modify engine parameters such as compression ratio and all that, making a conversion a bit of a hit&miss affair, AND introduces back the disadvantages of glow (flame-outs when you don't want them, for example, burned out filaments, necessity of field equipment). Ignition, especially on larger planes, allow for several 100% foolproof safety measures: Programming a fail-safe that closes the carb on loss of signal will not safeguard against wire breakage in the RX power supply. A proper ignition lay-out WILL do that. Glow does not safeguard against starting an engine while the RX is not powered up.

Your boxer should get similar fuel economy compared to your radial. Mine does about 45 minutes of normal flying on 250 ml, or about 8 safe towing flights (each consisting of roughly 1 minute of WOT on the way up, descent, low pass for line drop and a go-around for landing).
As a global rule of thumb, you can check the engine power with a propcalculator, then assume approx 300 grammes per kWh, or roughly 300 ml per hour per horsepower delivered. My Boxer clocked at 2,5 hp once, absolute peak I ever observed, therefore it should roughly consume approx 0,75 litres per hour at WOT, give or take. Never measured that to any degree of accuracy, but it seems consistent looking at the towing flights: I typically get 8 safe tows from a tank with about 20% reserve, so a consumption of approx 200 ml. The 8 climb-outs of 1 minute allready are responsible for half of that volume. Take in account the warm-up runs on the ground (cold engine takes 45~60 seconds to reach the temperatures where I am willing to give it WOT), the taxiing, the descent and go-arounds, and I'd say that is not too far off.

I have an Align selfcontained starter for those planes that have a difficult to reach carb (making a proper prime virtually impossible).