It is possible to change the ISOM section back to a solid type, but this probably won't give you what you want. (use the ISOM tool, write as integer, use 16).
The next version of StarForge is going to have a prototype isometric terrain mode, which does not depend on the ISOM section to work correctly.
When is it coming out? Possibly tonight, but definitely by midnight on tuesday, the 20th.