Sunbird Mods? 48volt? 60volt?

Hi All! Just picked up a Rad2Go Sunbird for $85. It had a broken battery wire and after resoldering it all up it charged up and ran like a charm (one of the batteries is a bit on the 'bad' side, but I am happy). The thing is slow as syrup, BUT, I love how it is completely (I mean TOTALLY) silent! I have about 15 different scooters, most chain driven, and all are like a herd of elephants compared to the Sunbird. So, I decided that I want to keep the silence, and hence the poor underpowered hub motor (250w, 36volt) and try and figure out how to put some more umph into it. Since it is setup with a smaller spare battery pack I figured I would transfer the current controller (which also controls all the other electrical: lights, turn signal, loud horn, etc) to run solely off the smaller battery pack and get a 48volt basic controller which would only drive the motor and get the throttle input.

Question: anyone have any idea about this particular hub motor being able to handle the additional volts (and possibly more amps)? I don't want to burn it up and being a hub motor it would be difficult to rig up a custom motor cooling system. Hub motors seem to be made pretty robust and I am hoping it can handle the additional power without burning out. Any suggestions for the controller (48v, ??amp) or any other improvements to make to this beast?

I appreciate your responses - thanks!

PS: Since 36volts takes the thing to 15mph, I hope to get 20mph out of 48volts - is this dreaming? Can I safely go on to 60 volts? What do you all say?

Also: The Sunbird has a freewheeling pedal and chain drive to the same hub motor.... is it stupid to rig that up to another motor for like 'high speed' - any suggestions in the arena? I have more than a few 500w/24volt motors and 800w/36volt motors. They all have 25# chain sprockets on then and I would need some sort of bicycle sprocket - any ideas here?

thanks again!