Its freshly rebuilt; theoretically no gear wear or selector plate wear to worry about
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But it wont stay in neutral (assuming it is neutral - std is 1 up 3 down) -that is a big worry
. I'd be taking off the end gearbox cover and with the engine off check the selector plate is nice and solid. You should be able to change gears with a screw drivers leverage, each gear including neutral should be positive, you can turn the back wheel to assist testing each gear. Ideally the box should have had this verified before putting into the bike.
These motors should not fall out of neutral. If in doubt you can strip it all in situ if you remove the clutch, its not a big job but it needs resolving.