Hi, you should update your profile so we know what part of the world you are in...
I wish you had mentioned in your first post, what you wanted to accomplish with your "mods"...? You don't mention your local terrain (hilly, flat, etc...). You don't mention your desired distance to travel. You don't mention how much you weigh...? You don't mention if you want speed, torque, reliability, or daily commuter.
I've had a few of these, and they are a "neat little ride" for the neighborhood.
Depending on your "needs & budget", I would recommend just getting better batteries than SLA's. You could go for Lipo, Lithium, or a LifePo4 battery depending on your skill level. SLA's are heavy, and are reducing your travel distance if that's an issue. I would pick up a second charger as they tend to unexpectedly die. You could use the 2nd one on your scooter for charging on the road/office/school.
I didn't see a huge difference from changing the motor from 800w to 1000watts, but if the motor dies, than you could contemplate it... (might need another controller etc....)
A volt meter, or a Watts meter are good/important tools to have.
Keep your money for a "battery upgrade", and left over money to keep the tires, brakes, repairs, and maintenance up to date.
I hope this helps, IAN...