You can put an internal ATAPI Zip 250MB drive into the MPC3000 if you use one of the ACARD SCSI-IDE bridge cards. That is the only way, as Iomega never made an internal SCSI 250MB drive. I can help you with this, so PM me if you want to pursue it.
Someone suggested disassembling the external model for internal installation - that won't work. The external models are built an entirely different way than the internal models and cannot be switched like that.
Yes, finding a CF Card Reader drive for the MPC3000 if you aren't running Vailixi 3.5 is like winning the lottery - you have about as much chance! Only the PCD-15B or PCD-25B models work and those are pretty darn scarce these days. If you are running Vailixi, you have a lot more options and some of those models are still available on fleaBay.