Several years ago I got a cheap combo drum set ordered from the Sears
catalog just to use for a band organ.  It came with a bass 20" diameter
by 16" deep.  I was able to cut down the hoop and fashion a new inner
rim from strips of wood.  As I recall, I was only able to cut it down
to 20" X 12" or so because of limitations in the drum hardware, which
I did not want to have to rebuild.

You might have to remove the material covering the hoop, if any, and
fill some of the holes in the hoop for the legs, the attachments for
the cymbals, etc. as well to get a nice appearance.  I painted mine,
but you might want to veneer it so it will have the appearance of an
old drum.

This is not a "quickie" project, but the results are quite
satisfactory.

Dave Vincent