KeyBinding question

I've got tired of lugging my 2010 vintage 14 inch Dell Latitude E6410
around for meetings conferences etc, (though it still works very well).

So I decided to buy a much lighter machine: 11.6 inch Stonebook Mini, which
has many nice features (including removable battery). But it doesn't have
separate PagUp/Down keys. Instead it uses Fn plus the Up/Down arrow keys,
though it reduces the pain by having a second Fn key on the right, next to
the arrow keys. (Fn+Left and Fn+Right emulate Home and End).

I can live with that, but since I have several rarely used keys on the top
row, I wondered if I could map them (e.g. F11 and F12) onto PageUp and
PageDown.

Ctwm is very flexible so I wonder whether it could handle this mapping.
Perhaps it is necessary to go deeper into Linux/X11 innards, e.g. altering
/usr/lib/kbd/keymaps/xkb/gb.map.gz (for a UK keyboard) or something deep in
the X window system, perhaps invoked via ctwm?

Any suggestions.

(I have the latest bzr Ctwm running on this machine, using Fedora 25).

Thanks

Aaron