Here is a hint:
If the cylinder has been removed from the crankcase it is easier to install the piston in the cylinder while on the bench. These little pistons are hard to get your fingers around.
Once the piston is in the cylinder, push the piston out of the cylinder only far enough to clear the piston pin hole. (make sure the piston is in correctly. There is usually an arrow on the top of the piston that should point to the exhaust) Put one piston pin keeper in the piston and start the pin into the other side. Stuff a rag into the crankcase mouth. You do not want to drop anything into the crankcase,
Slide the cylinder onto the studs just until the hole in the con rod lines up with the piston pin. Then push the pin home and install the other keeper. Then push the cylinder the rest of the way down.
This method will only work if the pin is a sliding fit in the piston and con rod.
Good luck. Let us know how things go.