Thanks everyone. I'll probably make some simple changes to the bike and see how it rides.
Currently it's setup 46/15, so maybe a replacement 19 tooth freewheel?
I was able to find an 8 speed IGH that fits in 120mm track spacing. Sturmey Archer
X-RF8 (W) , which may be the best bet if I find that I need gears. For now, I'll get the bigger tires, better brakes, and a better geared freewheel.