Install kernel and userland without source?

["J. Lewis Muir" <jlmuir%imca-cat.org@localhost>]

I have an amd64 router running the netbsd-8 stable branch that does not
have sources and does not have pkgsrc, and I'd like to build the kernel
and userland from source on another machine and then install them on the
router; how do I do that?

I'm aware of the "Updating an existing system from sources" chapter of
the NetBSD Guide

  https://www.netbsd.org/docs/guide/en/chap-updating.html

but it only explains how to install from source on the same machine.
It also has a section titled "Using sysinst", but I want to avoid an
interactive install; I'd rather just have a set of commands that I run
to do the install of the new kernel and userland, and I'd just write a
script to execute them.

That same chapter in the NetBSD Guide also references some tools
available in pkgsrc, but, again, I don't have pkgsrc (nor pkgsrc binary
packages) on the router.

I found these

  https://mail-index.netbsd.org/netbsd-help/2008/03/04/msg000089.html
  https://mail-index.netbsd.org/netbsd-help/2008/03/04/msg000090.html

but that's for upgrading from NetBSD 3.1 to 4, so I'm not confident that
it applies to NetBSD 8.

Thanks!

Lewis