I am planning to convert an exercise bike to a power generator ...
Hwere are some suggestions for permanent-magnet DC motors that -might-
work for you:
To experiment with drive ratios you could make wooden pulleys on the
generator and use several turns of cotton string for the belt. Tie a
bowline on one end of the string and adjust the length and tension
with a tautline loop on the other end. Then the generator mount
doesn't need to be adjustable. As a kid I used string and wood pulleys
on rock-tumbler (bird-scaring) windmills in the garden that ran a
month without maintenance.
Volt and Amp meters:
My order of analog current meters took a month to arrive from