Why would you need the MMU feature for?
As Jens pointed out the MapROM feature works without the need for an MMU. other than Amix - what would you need an MMU for?
Argualby the EC030 and full core 030 should be replaceable - I cannot see a reason as to why not - since the ACA630 is socketed - however there is a custom heatsink involved so it would best to seek confirmation from Jens.
LOL.... "don't even think about overclocking" - now does that mean its running at half that clock (i.e. 25Mhz) or is it running at the full burst of 50Mhz -