two or three options spring to mind...
you want a sprocket for either the honda c50 or a sachs madass, etc...just about any bike shop should have from 13t to 16t. stick to the small side for that bike

pretty sure every china engine uses the same spline, based off the honda... think about $15.
oh. be careful of the width... try to make sure its for 420 chain... or you may be fitting a new chain as well...
z50, braap, certain yamahas, motovert...its fun finding out who lists the same sprockets! not...
or theres ebay...any generic "pit bike" sprocket will do. again...12-14t, no bigger.
or a local pitbike shop.
or a wrecker. could get messy when you mention pitbike
youre gunna need the retainer plate as well... it screws onto the sprocket and locks into that groove on the shaft... never had to buy one, no idea where to find it...
a circlip will work as well...any old mechanic should have a box of them...
could be lucky with a file or something and rescue the shaft... but it looks ugly...
and no, it isnt a replaceable part...
or it is, but its a challenging job. split the engine, dealing with springs and things that pop apart...