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.