The mechanical principals behind a heat pump can't be that complex.
http://en.wikipedia.org/wiki/Heat_pump I would check the fluid level.
Then I would look around for a similar valve from a HVAC place and
swap it.

Replacing a reversing valve is a BIG job. First, they are soldered into
the "outside" unit. When you open up the system you will expose the
compressor oil to contamination from moisture in the air. It will require
a longer than average "pump down" to restore the system. The existing R-22
(or whatever) will have to be "recovered" and not just dumped into the
atmosphere.

The OP definitely needs to get a second opinion but this is definitely not a
DIY project. Considering the age, it might make sense to replace the
outside unit. His best bet is a one man shop type of operation.