if the cylinder dosnt have the correct compression that could be your crankcase pressure problem, if the rings are worn the commpression created by the pistin could go around it and build pressure in the case. the leaky head could simply be the seal around the compression release. as for your shifting, try adjusting the clutch. its on the right side case cover (as you are sitting on the bike) just back off the nut and tighten the screw until you feel resistance, then go back an 1/8 turn, then tighten the nut back up
1976 XL100
1978 Z50
1980 NC 50 Express
1983 ATC 185S (staightpiped)
1984 ATC 200X
1985 ATC 250ES
1985 ATC 350X
1986 ATC 250ES
1986 ATC 250SX