after looking at your scaler , you infact have a unique one it clamps onto the feedhorn unlike most scalers that the feedhorn slides in and out of the scaler. so with this information i suspect that you should be able to fit most all of them in it ( if you used the asc421 you would probably have to shim it)
if you measure the feedhorn ( the part covered by the blue cover) remove the cover and measure the feedhorn .
my chaparral scaler is 2 21/32 inches
chaparral feedhorn is 2 20/32 inches
bsc 621 scaler is 2 22/32 inches **
bsc 621 feedhorn 2 21/32 inches
** scaler has standouts to support the feedhorn.
in most cases you would have to grind a bit of the chaparral scaler for the bsc621 to fit (slide in) but with yours being a clamp type scaler you shouldn't have to just insert it.
but here's what i would think on getting the correct focal distance for your dish. your scaler is almost non adjustable so hopefully its close. then when you insert the feedhorn into the scaler it should all fall into place. so try your current lnbf see what the signals look like and we will work from there.