You're using a really old version of spectre. Whilst spectre is still in the IC5141 stream, since late 2004 there has been a separate stream for the analog simulators, called "MMSIM" which should be used instead of the spectre that's in IC5141 (in newer IC releases, spectre is not included).
If I use any version of MMSIM (MMSIM60, MMSIM61, MMSIM62, MMSIM70, MMSIM71, MMSIM72 - as you can see, there have been quite a few releases - one major release per year), the (* inherited_mfactor *) syntax is supported.
In IC5141 you could do a similar thing, but it used an older syntax:
parameter real (* integer inherited_mfactor; *) m = 1;
(* inherited_mfactor *) parameter real m=1;
which is the newer syntax.
So I would suggest you use a supported version of spectre (e.g. MMSIM71 or MMSIM72), but if you really can't, you should be able to change the model to use the old syntax.