How Difficult to Replace Valve Cover and Clinder Head Gasket

Only special tool you need to split the cases is the flywheel puller and a camchain breaker/rivetter. The rest you can do with normal tools. A torque wrench might be a good invesment tough.

Taking the head off is not that hard, just make sure you get the timing right when putting it back together. All the info is in the doc http://www.husaberg.org/wiki/index.php/Engine_rebuild

I did it and if I can do it, a monkey can do it especially with a little (lot ) help from the people on this site.

Work from the top down. Most of the oil leaks in the older motors originate from the rocker cover, the valve inspection covers and the breather hose. Later motors can leak from the water pump, the crankcase join and the counterchaft. Fix the easy ones first and then see if others really still exist.

When the rocker cover is off you can have a good look at the roller followers. These are a regular replacement item and life is a function of the orignal setup, oil used and how hard the engine is revved. Let them fail and it will be expensive. The problem area with the rocker cover is the front left corner. I used 3 Bond Grey and wiped a smear on the outside of the join as well to finally sort mine. Others have other techniques including making sure the surface is flat, a piece of plate glass and some valve grinding paste can do that, and other sealants.

Steve

I agree with Steve.
It's unlikely that there is a leak from the head gasket or the seal between the cases.
Up the top the rocker cover is more than likely the cause. Take it off and make sure it is perfectly clean before re-assembly. There is no gasket and they have a tendency to warp.
I have used 3 bond grey and Loctite blue siicon. Personally I think the silicon was easier to use as the 3 bond is a little too fluid and not as good for gap filling. Whatever you use don't go overboard, let it tack off a little before assembly. Then assemble and just nip up the bolts, wait a while (a couple of beers or so) and then tighten up.
Check that the valve inspection covers are flat. If someone has used brutish force on these then they will be warped and will leak. If not flat then get some emery and a nice flat surface and make them flat. Get some new gaskets. I don't use any sealants on the inspection gaskets as I like them to come off easily. I use a bit of grease on these gaskets, you could also use anti-seize.
Down the bottom end check the side cover gasket. More likely a leak from here than from between the cases.

BTW - Put your location and bike model in your signature

Coops

I got a '99 FE501. There are a few funnies with Husaberg,as any other engine has its own funnies! But what you doing is quite straight forward!
I just put all mine back together, this week. I used loctite{orage} gasket sealer. It replaces gaskets but needs mx of 24hrs to set between the faces. it never sets outside so you just wipe excess off. A concern is the screw threads on rocker casing, they are alluminium so strip easy. Hot tip visualise a spiral and tighten them from centre outwards until you happy they are all tight. My cam had 2 "E" clips on and I renewed it with a single spring clip. {No special tools needed, Just tiny screwdriver to prise them off.{ and a magnet if you drop it!!!} other than that just keep in mind what Steve is saying about those rollers, it is catastrophic when they go...

You must try it yourself...shops will rip you off.