Hello Gymjag.
Welcome to the site,
The crank you need fits the following:
1986 KX80-G1 KX80
1987 KX80-G2 KX80
1987 KX80-J2 Big Wheel
1989 KX80-L2 KX80 (S)
1989 KX80-N2 Big Wheel (S)
But, you can still get a rod kit from Kawasaki that fit's the following,
1984 KX80-E2 KX80 ('84 KX80-E2)
1984 KX80-E2 KX80 ('85 KX80-E3)
1985 KX80-E3 KX80 ('84 KX80-E2)
1985 KX80-E3 KX80 ('85 KX80-E3)
1986 KDX80-C3 KDX80(S)
1986 KX80-G1 KX80
1987 KDX80-C4 KDX80(S)
1987 KX80-G2 KX80
1987 KX80-J2 Big Wheel
1988 KDX80-C5 KDX80(S)
1988 KX80-L1 KX80(S)
1988 KX80-N1 Big Wheel(S)
Of course you will have to have your old one pressed out and your new one pressed in.