While alcohol or alcohol/nitromethane engines ("glow engines") won't run on gasoline without modification (fuel mixture too rich and may not be possible to lean out enough, glow plug incompatible with fuels that aren't mostly methanol or ethanol), any small piston engine can be easily converted to run on gasoline.
Tiny spark plugs are available with the same thread and thread length as a glow plug; spark ignition systems (classically, battery and coil, though modern designs will be capacitive discharge giving a hotter spark and longer battery life) are available. One would need to add a breaker point or Hall effect spark trigger driven by the crankshaft as well. For a .60 cubic inch (~10 cc) or larger engine, this is entirely practical, and even for somewhat smaller ones (I have a Madewell .49 cubic inch, appr. 8 cc, that was built in the early 1950s as a spark ignition engine).
The resulting engine will have significantly lower power output than it would when running on alcohol, never mind alcohol/nitromethane, but it will balance this with greatly reduced fuel consumption.
Smaller engines (less than about 6 cc, based on what was once available) will gain too much weight from the batteries and ignition unit, and may require more modification to mount the spark trigger (Hall effect is more tractable in this respect than breaker point).
Alternative to a replacement engine (attempting to get the necessary power to weight ratio to make such an engine work in a helicopter) you can, with a little work in sourcing materials, make your own glow fuel.
You need 91% or higher methanol; this is sold as a solvent in home improvement stores (in the paint section) for a few dollars a gallon (a couple dollars a liter), assuming it's not restricted in your location due to environmental or safety laws. You could also use 91% ethanol; it'll produce slightly less power but may be easier to obtain (liquor stores, as a last resort due to price; ethanol is also sold as a solvent, labeled as "denatured alcohol" or "methylated spirits" depending where you are). You need a suitable lubricating oil; generally glow fuel is made with lighter oil than the two stroke engine oil commonly sold for chain saws and leaf blowers (and older off-road motorcycles). Originally, glow fuel (and two-stroke gasoline) was made with castor oil, which you may still find at speed shops, but you will likely get the best results with SAE 5 weight synthetic motor oil.
Nitromethane is likely to be hard to come by; it may be available at speed shops, but due to "off label" uses it may not be sold to walk-in customers. The one time I've purchased it, in order to "boost" commercially made glow fuel that wasn't "hot" enough, it was dispensed directly into my fuel jug. Fortunately, most .60 cubic inch (10 cc) glow engines will run quite well on 0% nitromethane, staying cooler and lasting longer in service (you may, however, need to replace the glow plug with a "hotter" one if the engine was sold for, say, 20% nitromethane fuel).
Simple glow fuel, once you have the components, will consist of 20% oil and the remainder methanol or ethanol; mix thoroughly and store in an airtight container to prevent taking up too much water from the air (do this with your alcohol feedstock, too). With oils specifically formulated for model airplane engines (Klotz used to be a well known and excellent brand -- don't know if they're still out there, given the way electric has displaced glow engines), you can use less oil, as specified by the oil manufacturer. Lower oil will give more power and make less mess from the exhaust, but 20% is correct for castor or synthetic engine oil.