if it's not pissing water out the pee hole chances are good the pump is shot...
all the pump really is is an impeller that "sucks" water thru the intake vents on the lower unit and pushes it up into the head...
check this site out - may be able to help ya...
HERE!!!
or just go by a Johnson - the new ones are smaller and lighter so you could step down in size but still maintain the high output of the Merc... good luck