I can't find anywhere they provide the source code. Maybe it's on the disk img, but in any event it's going to be pretty difficult to get it working on ARM without the creators getting involved.

Also it seems like I've heard routing performance in OpenBSD isn't that great on ARM. It typically also has trouble using hardware accelerators that are commonly used on ARM routers to get faster routing performance, which is why a lot of people build them with beefier x86 machines. I don't have a lot of experience with it OpenBSD on ARM though, so someone who knows better please correct me if I'm wrong.