Welcome to the world of zipzaps!
Everything you want to do is very possible, and allready done with tons of tutorials to do it.
There are no specific springs you can buy for the zzmt, so you have to look around in pens and off of other things.
They don't make different chassis, or lift kits, that's for you to do.
Here is how you can make it slower, and still have lots of torque, it's called the 4th gear mod, found here :
http://www.gcrossett.com/zipzaps/mod...slow/index.htm
To make the tires wider, try fliping them around. Put them on backwards.
There are lots of crawlers out there, just search around the forum, and web.
Welcome!