Use a new gasket as it isn't worth the hassle of doing again, so 4into1 is a good source for the replacement gasket.
Make sure you get both surfaces nice and clean. I use an aerosol chemical carb cleaner when I dont have the Permatex gasket remover handy and a 3M scrubby. The trick is to spray the chemical material into a small container and use a small paint brush to apply a few coats of your chemical on the bottom oil pan cover sealing surface and the lower case area. Let it sit for a few minutes and put some chemical on the scrubby and be sure to wear gloves and start to scrub away. Reapply your chemical again and let sit and scrub and usually after a couple of treatments you have nice clean surfaces with no scraping or gouging of the surface material with metal tools. Pull off any large pieces of gasket if they are reasonably loose to begin with before you start the process.
I will also spray the new gasket with an aerosol copper gasket spray to assist the gasket in sealing, although this isn't necessary and just my OCD.
It also sound like you need to do a cam chain adjustment for sure and the clutch isnt set right if the bike creeps in first gear with the clutch disengaged. These should be looked at ASAP as leaving them will cause damage especially the cam chain being too loose. The cam chain is normally adjusted with the engine idling at 1000 rpm and loosening the adjuster bolt on the front of the engine case. This will only work if the adjuster horseshoe in the lower engine case isn't stuck or damaged from years of neglect. Worth a try as it is self adjusting if working correctly.
The clutch has an adjustment and spec noted in the service manual that you adjust the locking nut and screw adjuster per those instructions on the side case of the motor first and then fine tune the adjustment via with clutch cable adjustment on the handlebar.