Honda Dio AF18 Starter motor issues

[bootz1]

So I hadnt tried out the starter motor in the DJ1 replacement motor until tonight and its not engaging with the engine

IE the pinion isint meshing with the gear to turn the motor over but it whirs as it spins inside

Do these starters function the same as car starters? Is there anything that are prone to wearing etc? I havent pulled it out yet and may just leave it for now...

Oh and will early version AF18 starters work on the later version engine?