There are two fuel filters on a BX23.
On the 1st filter, the one under the tractor just before the electric fuel pump. the pressure you'll encounter is gravity induced but depending upon how fast you can swap the 1st filter (the one before the fuel pump) you could lose a 1/2 pint to a gallon of fuel
That said, since the fuel line is rubber, some folks have used a pr of vice grips and/or hemostats (roach clip for you children of the 60's ) to seal off the line about 4 nches or so before the filter, this should stop or slow the fuel flow enough to change it out without much lose in the gold-plated fuel.
The 2nd filter which is located in the engine compartment just before the fuel injector pump will leak some too but not as much as the 1st filter.
Good luck
I suggest that you clamp the rubber fuel line near the first filter, as explained by Volfandt, and then proceed to replace both filters. You can also use some newspaper to catch the small amount of spillage.