With regards to aftermarket engines, there are 2 methods in which a golf cart can be started and there are pros & cons for both.
A key start setup completely bypasses the factory starter/gen and uses the onboard starter on the engine that is being installed. This means that you wont be using much (if any) of your factory wiring and your engine will idle/stay running after it has been started or until you turn it off. This also means that you will likely have to "wire" your new engine from scratch or buy a pre-made wiring harness.
|-Simplifies engine bay||-Have to re-wire|
|-Can fit a much larger/more powerful engine||
-More carburetor tuning involved
-Uses more fuel
-Less top speed (belt has to be loose to idle and shift)
A pedal start setup reuses your factory starter/gen and wiring to turn the engine over whenever the gas pedal is engaged. The engines idle screw is backed out so the engine will not idle. This method does not require any additional wiring but typically the only engines that will be pedal start are the smaller GX390/Clone engines and the factory engine. Most other aftermarket engines are too large to also incorporate the Starter/Gen and retain the pedal start.
|-More top speed (tighter belt)||-Adds complexity|
|-Less Carburetor Tuning||-Can works with smaller engines|
|-More fuel efficient|