I used to use TOR, advanced editor made by Leonid Baraz. There's an English version in the archive at the second link Markku provided.
I also saw ROM editor called "MSX-write", I can try to get image if you wish.
It just needs MSXDOS.
Add COMMAND.COM and MSXDOS.SYS to a disk with this file and type SKMSX when in DOS.
And make sure you are in SCREEN 0 and not SCREEN 1, else it doesn't work correctly.