I like the lids. It has the purpose of keeping dirt, lint, and airborne junk out of the top. It won't fall off unless you abuse it. I've got a 401 and two 500s and the tops have NEVER come off.
Here is a possible source for a top:
The Sewing Center
2504 S. MacArthur Blvd.
Springfield, IL
Phone: 217-525-7744
Steve Sutsser
I asked him yesterday about the top and he said he might have one. You'll have to contact him to find out.
Besides an incomplete machine just don't look right.
Joe