Hello there!

The main thing to consider is; is someone who knows what they're doing doing the work?
If this "mechanic" doesn't know NSR250's inside out you'd be no worse off and learn a lot more doing it yourself, nothing on a 2 stroke is even slightly complex but NSR's have quirks and there's more knowledge here than probably anywhere outside of Japan.

Hunt back though the forum and there's been tonnes of problems with the 300's seizing, all traceable to poor setup and assembly errors, don't join them.

Also more than one person here has fell foul of taking their bike to an engine place/garage and they've made expensive mistakes (crank in wrong, wrong crank bearings used) or just plain annoying mistakes (sump plug not done up properly, bolts sheared off and not mentioned).

The '18 is getting on a bit now and that's it's weakness, Honda life the cranks at around 30,000kms and there is no such thing as a new mc18 crank.

It's not the best check but get the fairing lowers off and get the flywheel cover off (left hand side of the engine as you sit on the bike) it's the big bit of plastic held on by about six 8mm bolts, your flywheel will be bigger but you'll see this, now the round thing that spins when you push the kickstart, try to wiggle it, if you can feel any movement at all the 300 kit is the least of your concerns and unless you know of someone local who rebuilds MC18 cranks you'll be wanting to talk to Andy.

Well you did ask!!
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Please do not PM me technical questions, if you can't find it on the Forum start a thread