Nice program, it works well and I like it :) .
I don't know if you use Prompt for space-saving (the program is only 140 ) but I would use Input "Number?",N to make it some more userfriendly.
I also have put in a check for N, if N is bigger than 3999 the program starts using the wrong tokens (4000=Mv).
And, I don't know if this is just the engine, but of course some more things can be done to make it more userfriendly. Like a ClrHome in the beginning of the program or a Disp "Roman number=" at the end.
I like the program very much, thanks for making :D