I know one of the chaps who wrote First Word Plus for the Archimedes. He'd moved onto other things even by then, so wasn't available for doing the rewrite. I also remember it being so dreadful it hurt
RISC OS 2 (and even RISC OS 6) can run most Arthur GUI apps still, but only single-tasking. druck's right; to take advantage of any PMT, everything would need rewriting, just like everything did to take advantage of CMT in RISC OS 2.