The weep hole is there to drain water in the case the seal goes bad. Hard to say why it went bad.. Could be a bearing wore out and caused the seal to go, or the seal just wore out.
On cars it is cheaper/easier to replace the whole unit, and the parts stores usually take a core. I'd imagine it is the same with tractors.
Looking at the parts diagram, they sell it as a unit or parts but the impeller looks like a press fit so it is probably not serviceable w/o replacing everything anyway.