I have used the 23cc ST engine with a large 18x8 prop, and it was very powerful at 7200 rpm, not in rated HP, but in real life performance.
HP ratings are a paper tiger. The MVVS gasser is rated at 3.8 hp, but these are factory test data with tuned pipe, set to best power, not best fit for use. In real life, operational hp of this gasser is about 3.0, slightly less than the operational power of the OS (prop 18x8 at 8200 for mvvs26 vs 8400 for the OS26 and this prop is too small at 9200 for the mvvs35). In switching to gas, a slight weight increase has to be accounted for as well. The tank can be smaller, but that does not completely compensate the added weight of ignition and battery.