Pretty Sad feeling when the blower wont blow isn't it?
They will continue to work inadvertantly until the pump is replaced but when it comes to the time you have to pay for it go with the Johnson pump far superior to the Ford one in my opinion.
Mine is still under warranty but when mine went 6 months ago I chose to pay for the better choice rather than taking an inferior pump that will also go in a short time plus if you plan on doing any further "mods" you will need the extra volume the Johnson offers.