Here's a simple but effective text editor for the Commodore 64 from the Starter Kit disk by the NorthWest group of California.  Although it's calling itself a "Word Processor", it's really a line based text editor.  That aside though, it does the job, and supports load/save from disk or tape, and printing among other things.  If you just need something simple to edit a .SEQ file, this might be one to check out.

This archive includes the full NorthWest Starter Kit.  There are some other utilities / programs you might find useful on it.