the only gas engine that comes very close to 1.60 FX powerwise is the MVVS 1.60 (about 90% power). The modern ICU ignition uses LiPo ignition batteries, so the weight disadvantage becomes real small.
For flight, a 10 oz fuel tank will do. This cancels out the extra weight of the ignition. Add to that the lesser vibration of the gasser, so you need less vibration isolation, and the difference gets even smaller.