The speed sensor serves 3 purposes, its provides for the 180km speed cut, it tells the computer when the engine is at moving with no throttle or stationary with no throttle, and it stops the engine check light coming on.
Sooo
The 180 speed cut is no problem
The no throttle thing is not really a concern - except if you are cruising along the road and remove your foot you can often get a little bip on the throttle from the computer - think it is to stop stalling as the revs drop towards idle.
The engine check light is a pain, it is a diagnositic code to tell you the engine has been reving at 3,000 while stationary - of course you can choose not to connect it or take the globe out - or you could put a cruise control pickup on the tailshaft like I have and feed into the computer. It you only use one magnet you would have to be doing 4X180 for the speed cut to occur and everything else will work as old man toyota made it.