There is a curious side to this:

In CL Stunt, the Old Time events include 'spark ignition' with a 10 point score bonus to make up for the 'difficulty' of getting a spark engine to run consistently and well. (Diesel engines get a 5 point bonus, too.)

Most of the suitable engines for this event haven't been manufactured for dozens of years, although RJL in Califonia does offer several replicas of 1940's spark engines. Coils, plugs and electronic ignition devices are available. (The electronic circuits 'switch' the spark timing using a very low current through the mechanical breaker points, triggering a higher current through the coil to the spark plug. That saves a lot of spark erosion at the breaker point surfaces.)

Gasoline is a lot cheaper than glow fuel, but you still have to run at least 25% oil to keep the engine going. If you use a petroleum-based oil, it needs to be about SAE70 weight racing oil, NOT a gear lube. Such is fairly scarce, and not as cheap as ordinary engine oil. Castor works well.

Gasoline fuels don't produce as much power as methanol-based glow fuels. Even though gasoline produces more heat per unit of weight, it has a trickier fuel-air ratio tolerance than methanol. The amount of air the engine can "process" is limited by its displacement. Methanol can burn productively at a much richer needle setting than gasoline, so we CAN burn twice as much or more in the same volume of air the engine uses at a given RPM. ...Which gets at least the same power output...

Mixture settings for a gasoline/oil blend have to be leaner - consequently more critical; and since the oil is a percentage of the burnable portion of the fuel, a higher percentage of oil seems indicated to keep the engine lubed. Using twice (or more) as much glow fuel over the same time at the same RPM, if the same % oil were used for both a gasoline and a methanol fuel, the glow fuel washes twice (or more) as much oil through. That's better for the sleeve/piston and bearings...

Methanol also chills considerably when it evaporates, or even sprays, out of the needle valve. And since we'll use more of it, there's more cooling advantage for the engine.

Summing all this nonsense up, most CL Old Time Stunt spark fliers are using 5% to 10% nitro glow fuels. The needle settings are MUCH more tolerant than with gasoline/oil fuels. And the fuel may be the same as used in older, iron piston/steel sleeve glow engines. No need to carry too many different kinds of fuel.

Haven't seen info on the Magnum 0.50 cu in spark engine, yet. Will look shortly... IF it is made with the same approach as so many weed-whackers, etc., these days, it MIGHT JUST operate on around 50:1 with a special oil, as those engines do.

The airborne weight with modern batteries should be much less than with the traditional lead-acid, Ni-Cd, or NiMh cells - IF the voltage output is compatible with available model engine coils. Coils are likely to go 2 to 3 oz. If setting the engine is easily possible, the weight saved by using less of a gasoline/oil fuel for the same flight time could help reduce the weight penalty to almost an even match.