When I was scanning through back dated subjects I came across a question concerning the possibility of fitting a sealed cartridge bottom bracket (BB) to the C5. At first I thought this would be a good idea until I started checking the ins and outs of the matter. I now consider this to be a totally impractical idea.
Why impractical? The C5 open BB spindle is a special one off item, the spindle length being 140mm as opposed to the 119 -122 or so millimetres of the normal cartridge type. The reason for this is the spindle must be long enough so the peddle cranks clear the body shell which is not the case with normal bikes or trikes. There are longer spindles, but these are still short of the 140mm mark. Plus the extra length is solely for special wide BB shells.
You could of course cut, chop and weld all sorts of BB spindles to achieve what you want, but they'll never stand up to the strain that will be applied to them.