You could always replace the condensing unit with a newer one. Just get the information off the condenser (model # / serial #) and google the manufacturer. They are not that terribly expensive and you will have a much more efficient walk in than if you used a window a/c unit.
Sources for replacement condensing units can be e-bay, craigslist, graingers, r.e.michels co. and so many other HVAC suppliers.
I have been in the HVACR service and sales business for over 25 years and suggest replacing the condenser. You may need to hire an HVAC tech to replace the unit or maybe get a friend who can help you out, but the overall results will be better in the long run.