Tomos Nitro 50 Transmission

[zink71]

I have a 2011 Tomos Nitro 50. I stored it for winter in a shed. Mice may have gotten in it. There was either transmission fluid or oil that had leaked from the scooter. I changed the oil and I changed the transmission fluid. Scooter starts fine but as soon as I give it gas, it just makes a squel and the back tire does not turn. Also, I am not sure if it is holding transmission fluid as a lot of it has leaked out the overflow valve. Can someone give me next troubleshooting steps ?