HELP Crank Seals

Hey all, I am new the the forum and new to atvs. I just picked up a 1994 Polaris 400l 2x4 2 stroke. The guy before me had just rebuilt the whole top end. Problem is it won't idle at all. Runs fine at 1/4 throttle up to full. I took her home pulled the carb and intake manifold cleaned em up nice put it all back together and it runs the same. I thought maybe the intake manifold had a leak so I sprayed some brake cleaner around it and nothing happened, I sprayed and missed the intake manifold and the spray went down by my clutch and the engine revved. Seems to be popular opinion that this mean bad crank seal. So here is my question. How do I replace these? Do I do both sides? or just the one. If I only replace the one can I do it without splitting the case? How much is a shop going to charge for something like this? Should I do anything else while I am there? Is there a step by step on the process or maybe even a manual? Thanks for your input.