The power supplies don't match up.
In general, with compact Macs...you can safely swap the following boards (this does not factor rear housing differences or country of origin...)
128K, 512K, 512Ke, ED (any model), Plus all use the same PSU (although later models are more reliable--if you're planning on actually using a pre-1987 model of one of these and don't want to mess around with refurbishing it yourself, find a good working model from a platinum Plus).
SE, SE FDHD/SuperDrive, and SE/30 all use the same PSU.
Classic, Classic II use the same PSU, but if you're going just on analog boards alone, be aware of Revision A versus Revision B. The CRT connectors aren't the same (see the stickied thread about this). In general, Revision B boards are more reliable.
In general, with compact Macs...you can safely swap the following boards (this does not factor rear housing differences or country of origin...)
128K, 512K, 512Ke, ED (any model), Plus all use the same PSU (although later models are more reliable--if you're planning on actually using a pre-1987 model of one of these and don't want to mess around with refurbishing it yourself, find a good working model from a platinum Plus).
SE, SE FDHD/SuperDrive, and SE/30 all use the same PSU.
Classic, Classic II use the same PSU, but if you're going just on analog boards alone, be aware of Revision A versus Revision B. The CRT connectors aren't the same (see the stickied thread about this). In general, Revision B boards are more reliable.