What Crayford focuser to buy for a B&L Criterion 8000?
Item B is called a visual back.  The new focuser would just replace this, and screw onto item A.  The focuser you link to is designed for a refractor, but you need one for a SCT, like this
https://www.firstlightoptics.com/skywatcher-focusers/dual-speed-2-crayford-focuser-for-sct-telescopes.html
There are lots of different makes and prices.
Regarding the sizing, I'm not sure about an old scope like the Criterion: it's probably the same as the other popular current models, but you might do better with cloudynights.com to get that confirmed.