I just tested the build. It seems that the NetBSD build tries to compile for armv4 (which, as on Linux, means StrongARM only).

When trying to specify -a earm (which should imply ARM v3) instead of the default -a earmv4 to build.sh (./build.sh list-arch gives the supported architectures), the script complains that "MACHINE_ARCH 'earm' does not support MACHINE 'acorn32'".

So it seems there's more work to be done for the armv3 machines, sorry...

Update: Early Acorn machines with ARM2 and ARM3 processors were supported by NetBSD/acorn26 (see https://www.netbsd.org/ports/acorn32/faq.html). This is a bit strange, since IIRC ARMv3 got rid of the 26 bit PC mode.

Unfortunately, support for this port ended back in 2018 with NetBSD 8.0 and the acorn26 supported platform list doesn't include the ARM710 RiscPC, so it might also be significantly more difficult to get NetBSD to work on your machine...