Most part of Nucleus source codes are written by C. What you should do is to modify the just like hardware relation code(another word: BSP) and some kernel raltion codes(timer, interrupt, etc).
BTW: Nucleus has supported the ARM7 a long time ago. You could buy this version. Porting from 68k to arm7 is not an easy job.
There are several Nucleus ports available for ARM7. You might want to check with Accelerated Technology and see if they already have the port you are looking for.