I have a ms440 stihl piston in the cylinder is that a good or bad choice
Unless you measured the crown height and verified the port timing and squish gap, bad. Very bad. That could very well be why you have such terrible performance. Even if the squish gap works out ok, the shape of the piston top for most Stihl saws is flat, and the chamber of the YD is meant to work with a domed piston. Either way you are probably having low compression and the port timing is even worse than it should normally be. Generally speaking a bog stock YD100 with a decent quality cylinder should be able to hit 30mph with a 44t rear sprocket without trying. Heck, a healthy, well built, and decently tuned 49cc engine for these bikes can usually achieve a top speed of 30
The YD100 has never been known as an engine with good top end speed. In reality, despite the extra CC's they have close to the same top speed as most "80" engines. What the YD100 has in it's corner is extra "tractor factor" which makes them a better hill climber. Essentially, same speed, more grunt.
With your parts combination, assuming you have a phantom bottom end with a 12mm wrist pin and a YD100 top end your best match for a piston with be a 12mm pin YD/LD100 piston.
CDH actually sells a complete windowed piston kit for that purpose.
YD/LD 50mm Piston with 12mm pin here.
(if you have an older phantom bottom end with 10mm wrist pin, then you just need any ol' YD100 10mm piston, and those are everywhere)
If you still have your phantom 85 parts, you will need to use the reed block that came with the phantom to use that piston I linked above. If you dig around there are non-windowed pistons out there that would not need the reed block.
If you are already using that reed block on the yd100 top end without a windowed piston, that could also contribute to the low power you are making.
Nobody here is under any illusions that these engines are in any way high quality. They never have been, and even the "best" ones still come with their share of quality, manufacturing, and design issues. The Zeda 80 is probably the "best" of them all in those regards, and even they can be prone to some of those issues. You have to keep in mind that you are buying a whole internal combustion engine for basically pennies.
If you take the time to learn and apply what you learn you can easily turn these engines into something decently reliable, but they will never be as reliable as a modern 2 or 4 stroke engine. At the end of the day, these engines are kind of intended to be a bit of a throw away item.
