We have just received our first shipment of Mitrpak T-102-M right angle drives customized for hpb use.
The drives have an increaser ratio of 1:2, and have crank posts that fit J.I.S. cranks (all unicycle cranks and most standard bike cranks). The output shaft is 12 mm, with an offset groove for easy and inexpensive cotter pin attachment to your shaft's coupler.
These gearboxes have many bolt holes for different deck attachment orientations, and are rated to at least 75 in.-lbs of torque. Rick Willoughby pushed his one-off version to the breaking point, and Mitrpak has slightly altered the pinion/gear interface to strengthen it a bit.
You can find details at www.marcopoloboats.com. The photo shows only the standard T-102-M, not our customized version.