My 99 forester just had head gaskets replaced and now there's an oil leak coming from the crankshaft seal on the front of the engine.How difficult is this to change and what other things should I be looking for?its a 2.5 s.o.h.c engine

I don't know much about the engine you have but I imagine at a minimum you're doing everything involved in a timing belt change.

Basically it's the same as timing belt for disassembly. I highly recommend using oem replacement parts for the crank seal, there is also an o-ring that is between the oil pump and block. Take the time to remove the oil pump and retighten the screws that hold the back plate onto the pump as this is a common cause of low oil pressure. And replace both the main seal and o-ring, gray rtv is all that is needed to seal the outer body of the pump to the block.

Same disassembly as doing the timing belt, if that hasn't been done then I would greatly suggest doing it while your in there and I second the advice on the oil pump while your there. Anaerobic sealant on that oil pump I suggest and hopefully they did cam seals on the heads when they did head gaskets.