please help!! does any body know if a 400ex's cdi/rev limeter will fit on my 300ex.

i was going to buy my friends cdi/mod box but im not shure if it will fit.

he is dropping it becsuse his is like 14 and his dad doesent think he can take the switch between a ttr125 and his modified 400ex.

and if it does fit how much is it worth i will find out who makes it later, i just want an average price.