Using the above as a guide, it took me a little less than half an hour to do the first conversion. The .9mm Schmidt mech (which we do have) will fit into the Perfect Fit, with minor changes to the length of the mechanism.
I also drilled out the nib and made the shoulder farther down---this may not have been necessary, as the real problem was the shoulder on the pencil insert. That DOES have to be removed for the transmission to screw back in place.
When you are finished, the transmission will depress the top of the insert, allowing the lead to advance, when the "pen" is in the extended position. When in the "withdrawn" position, the pen transmission is not engaging the pencil mech, so everything writes normally.
Pretty cool,
Ed
YMMV (depending on how well you adapt to mechanical problems)