It's only a 1 speed
I would drain what ever oil is left in the case and fill with a half a quart of Mystery Oil.
Run the engine for 5 to 10 minutes to flush every thing out (don't ride the bike just rev it on it's stand)
With the oil still hot drain it out.
refill with SAE 20 or 30 "NON DETERGENT OIL"
This should clean things up.
The morini motor is a good strong motor that can take some abuse.
You might want to disassemble the carb and let it soak in some dip.
I use Bare Concrete Rust and Etch remover but only let it sit for no more then 10 minutes in this stuff it works great.
Good Luck and Have Fun