Stihl 051av eating chain oil

[phpreto]

I've got an older stihl 051Av that I use to run a 30" alaska mill. Problem is, it has a gasket leak from the chain oil reservoir to the crankcase, fills up crankcase with oil when sitting and smokes when running. How hard is it to crack the case and replace the gasket? I've considered gas tank sealer or something similar as a fix. When I pour gas in the oil tank the idle slows and saw tends to flood out, so its not a pinhole..
I'm an accomplished mechanic, just don't have a manual and not sure if I need special tools to crack the case. Any instructions/ideas appreciated.