Something else to remember is that the Tecate-4 motor is a counter-balanced engine. Like BigGreenMachine said, it's would take frame mods to switch to a 2nd generation power-valved KXT or KXF engine.
If I was going to that, I believe I'd try the KXF Tecate-4. You've got to mod the frame anyway. The KXF should be smoother due to the counter balancer and it has a 6-speed tranny. It is the most "evolved" Tecate motor.
1986 Tecate
1984 Tecate
1985 Tri-Z
1986 ATC350X
1985 ATC250R (2)
1985 ATC125M (2)
1982 ATC70 - Original Owner
175 TriMoto Frame with Pro-Tec Rear Suspension
3-Wheelin' since '82