I am also hoping to find the Center/Intermediate belt pulley assembly for a Forbes Drill Press FDB-16mm 12 speed which looks similar to this:

Intermediate pulley aseembly.jpg
All I know is that the shaft on the idler/center pulley assembly that mounts to the head of the drill press is roughly 13.5mm in diameter based on my caliper measurements of the mounting hole. The rest of the pulley dimensions are unknown to me at this stage.

This will be for a drill similar to what is shown in the photo copied from a members post on the woodwork forum:

example similar drill press.jpg

It maybe a long shot posting it here but most wood workers tend to not need the lower speeds and simplify the belt changes by removing the idler/center pulley assembly and converting their drill presses to a 5 speed or in some cases do a 3 phase/VFD conversion.