The majority of engines that look like yours are 47cc or less. The only engine I've seen with an electric starter is a Mitsubishi TLE43 "clone". I'm presuming that this is your engine, and it produces 2hp.
Your engine has a centrifugal clutch. It allows the bike to sit at a stop with the engine idling.
Actually, you seem to be getting EXCELLENT performance from your MB. When geared for flat ground, ALL engines your size have limited hillclimbing performance. JMO, the simplest and cheapest way to improve hillclimbing is to install a larger chainring sprocket and pedal-assist.
It's hard to help you if you don't provide specs.