Re: Piston Stuck
If you just installed a new jug and piston it's not going to have the same power as before until it's run in for some time, usually about 2 or 3 tanks of fuel before it loosens up and starts to make more power, but it shouldn't be a total weekling during this break in time either... did you check the compression after you got it all back together?
Also, this "tinging" sound you're talking about... does it do it all the time or under load? It could be detonation if you're using regular fuel instead of premium, or it could even be the piston hitting the head, and they also tend to get a tinny sound and lose power when overheated. the worst thing you can do to a new engine or new cylinder/piston is to run it too long without letting it cool down or try to run it full power for anything more than just a short burst.
I would pull the head back off and inspect the top of the piston and the combustion chamber for the tell tale signs of piston to head contact, and with the piston at bottom dead center, check the inside of the bore for any scratches in the plating or chunks of missing plating. These cylinders are usually pretty rough right out of the box and one should at minimum chamfer all the ports to prevent ring snags or prevent the plating from chipping off at the port openings. I've seen plenty enough new cylinders that the plating extends past the port openings and this will cause it to begin to peel off.
Now back to the piston to head contact thing I mentioned, it shouldn't happen normally but if the base gasket or head gasket is too thin or not being used, there is the possibility as well as simply putting a new jug on sometimes it needs a little extra shimming at the base as there is a chance that the seller replaced your engine with one that's not exactly the same or the engine in your bike has the 40mm stroke and he replaced it with a 38mm stroke engine so it's worthwhile to check your piston top and combustion chamber, as well as seeing how far up your piston is at top dead center.
I've had this happen when I put my original piston in a new bottom end and had to shim the bottom end and bring the jug up by 1mm to prevent contact as well as get the proper squish clearance. These things need to be checked anytime you put a different cylinder on because these can be that different even from the same seller.