www.hostelshoppe.com is "in your area" sort of. They have a good web site and
paper catalog. Light mounts are made that can mount to the head post. Any
cyclometer can be adjusted to 20" wheel size. The manual should have an algorithm
telling you how to esimate the constant that is entered. It is commonly but not
always wheel circumference in millimeters with fudge factor for the flattening that occurs. The bike pictured will have almost all the weight on the rear though. You may/will need a longer cable: some companies make a tandem wire kit which is usually long enough or it is no big deal to splice in a few feet of wire, 2 conductor 20 to 24g stranded wire is suitable. The mount will be a problem. Not sure how that is done with no tube that is at right angles to the rider. Good DIY project to mount it to
the tube between the legs. You would want a cyclometer with large number size
on the display. Steve