You can tune and customize the bike for top end speed or for low end torque but you have to pick. The 48cc motors stock preform very closely to the stock 66cc variants. Supposedly the difference is between the two is a half horse at peak power. That's 350 watts, its really not that much compared to what it actually takes to move a bicycle. A healthy young man that rides often should be able to put around 160 watts into the pedals continuously at a so called "touring pace." That will make up the difference and get your blood pumping in the cold weather.
That extra 2mm or so of bore between them isn't significant. If you're having trouble with torque, gear it lower and take life slower. You're skin and bones, muscle and blood not a metal machine: a bad fall will ruin your body faster than you know what's even happening.
Money is tight for most and time is precious so I don't expect anything from you but you don't have a front brake.