Originally Posted by Zug
It looks like you might have room behind the seatpost for a jackshaft to the crank spindle. Then add either a NuVinci CVT hub (135 mm) or a cassette and derailler. Both would probably require respacing the dropouts. A Sturmey-Archer 3-speed @ $99.95 (or 8 speed for $169.95) might not require respacing at 115 mm.
NuVinci CVT's go for almost $500.
If you've got lots
of bucks go with a Rohloff 14 Speed SpeedHub. Strong - torque loads of up to 250 Nm on the crank - that's 150Kg on the pedals. $1295.95 for caliper/cantilever brakes. Out of my budget.
I need a sugar momma.