Street Fighter 2 Turbo was never released for the X68000, I assume you mean Street Fighter 2' (Champion Edition in the West)
To activate 31kHz (VGA) mode in Strider:
While loading, start with the System Disk in drive 0. Soon the game will stop and ask you to insert disk A into drive 0, and B into drive 1. When you do it will finish loading.
Press a joystick button to bring up the main menu from the opening demo or intro. Press down once and press a button to activate OPTIONS menu.
Press DOWN four times, and RIGHT once, then DOWN twice more, and press the button. Presto, VGA-happy.
I don't have StreetFighter handy to check, sorry.