This is a not quite simple conversion that can be made simpler by keeping the pistol, but then most people are unlikely to notice the conversion.
I'm not all that happy apout how I managed to build the bow-part of the handcrossbow, but I sort of lost my temper after 3 failed attempts to glue the pieces together.
The decision to convert this model was as easy.
Coming from a AD&D world I don't like gunpowder weapons, and in my opinion the pistol was held wrongly on the side. This might look good in gangster movies, but with the old gunpowder weapons you wouldn't invite a misfire shooting that way.
Besides I like the hand crossbow.
Using a powertool (because I has 2 of these and wanted to use one) I cut the arm with the pistol just above the glove. This was the best place due to folds in the cloth.
I cut the cock from the pistol but decided that this crossbow pistol would use a hollow barrel (some crossbows did that and it protects the missile from harm until the shot is fired). In order to let the string getting into contact with the missile, I carved a groove along the barrel where the steel meets the wood using a sharp knife.
I made the arms of the crossbow pistol from a metal rod I bend into shape and then filed down in the ends to look like the ends of a bow. The tricky part here was getting the two arms to look alike.
Then came the hard part. The part I did poorly, but just didn't want to use more time/tries on: Attaching the bow to the pistol. After trying to glue them on, I finally used Green Stuff and filed it down afterward. What I perhaps should have done was filing the bow and the underside of the pistol in order to get a better grip to glue on.
Pining the arm I then glued it back on just rotated enough that the arms of the crossbow pistol was horizontal.
Finally I used the remaining Green Stuff to make a quiver to hold the quarrels: Roll the Green Stuff into a ball, flatten it and press a knife against it all the way around to make a lid. Then the quiver was pressed into place until it stuck. Very simple to do.