Alternatively, you could just connect Agnus' _VSYNC output to the _TICK signal - that's what J300 does.
Using _VSYNC as _TICK raises problems when you change the vertical scan rate, but the OCS Agnus can't do that anyway.
You'd probably want to disconnect (=cut) the appropriate trace to the PSU to make sure _VSYNC and PSU _TICK are isolated.
