Hello!
Are you sure you have enough power in your 12v motorcycle battery?
And y use a Sullivan Pylonbrand starter (or something like it) Avoid Kavan starters as these don't spinn fast enough.
You should use at least 7Ah or preferably a 10-12Ah 12V lead acid motorcycle battery.
The LA engines are a bit stiff at first due to their ABN construction but as soon you have got it running everything will loosen up.
Just reverse the rubber cone starter on the Sullivan starter and it will be easy to start the engine (but a aluminum prop nut will make starting even easier)
Put on a 10x6 APC or a 11x6 if it is intended for a slowflying trainer aircraft and just run the engine.....let it rew...not go slobbering rich!.

Regards!
Jan K