Quoted from an e-mail by Kent Hansen:

Uhm, I wrote a level editor which can edit all levels in both Mega Man 
I & II about a year ago. :-) I've attached the .ZIP file to this 
email. It works like most of my other level editors... Change level 
with F1, save with F5, move around with arrow keys/page up/page 
down... Switch tile sets with TAB, draw with left mouse button, pick 
up tile under cursor with right mouse button. That should be all, if I 
remember correctly. The reason why I haven't released it is because 
the sequence of the tile sets are a bit messed up (taken straight from 
the ROM though), and I never bothered to fix it... But it should be 
fully workable for most levels anyways.