Are you sure that 5.25" drive is only 20MB? Typically the 5.25" form factor drive was for the 120MB model (this is based on Apple service manuals). I am really surprised to see 5.25" 20MB drive as Apple had a deal on the 3.5" drives they were putting in the SEs at the time. Hard to believe they would go outside their deal, unless demand was that high.

As far as replacing it, have you removed it yet? The housing was designed to accommodate both standard size drive formats, so it may be a simple matter of using the same mounting screws in a different set of holes, without any additional hardware. I have the manuals around here somewhere ... will have a look to see what's required.