Get a Kavan reduction gear starter. A buddy borrowed mine to start his 180 4 stroke and it kicked it off with no problems. I've gone electric so I sold it to him and it's been starting all last summer with no problems turning it over.

I don't know if it will start anything, but who cares! It's too pretty to use on a dirty old airplane anyway!
http://www.rcuniverse.com/market/item.cfm?itemId=381745

I have a Sullivan high tork starter and if the field battery gets low it struggles to start an OS 40la. I charge it every few weeks and it will start almost anything. I fuel up the plane, and turn it over by hand a couple of times{not priming it, just checking for problems} then move to 1/3 throttle attach ignitor and use the starter to spin it up and draw the fuel an it starts. Less chance of overpriming and breakage. Different things work for different people and I'm deffinatly different.

The voltage will give more speed, but you still need quite a few amps of current to keep that torque up. If you have the voltage but inadequate current the starter will turn real fast, but the second you touch it to your prop it will stop.

I'm not sure if you are challenging this method of starting larger motors or not. If you add voltage to any motor and leave the winding or poles as they are then you will add both speed and power (torque). Sure, if you add more volts from a battery that has not got the capacity or current discharge rate to supply the load then you will just turn faster for a moment and stall when touching the spinner. I am assuming everyone knows to use 7 Ah or better batteries for starters.

The fact is my starter will not turn over my 120 2 stroke on 12v. Series in another 6v 7Ah battery and it turns it over very well.

I've started my DA 50cc engine with my hobbico "180" starter (that's the bigger of the two), using 24volts.

I wouldn't use 24v on a smaller engine, I'd be worried about bending up the conrod if the engine flooded.

One thing to keep in mind is that most of the gel cell batteries sold at the hobby shops aren't rated for deep discharge, and they can quickly wear out if you don't keep them charged. They also often age quickly, and while they seem to be ok on voltage, they just don't deliver the power that they did when new. So if your starter isn't cutting it, try another battery.