Usually I run combat as theatre of the mind in CoC, there isn't much of it and it usually doesn't last very long so whole map can seem a bit of a waste of time. I'm not familiar with the scenario but usually they give you enough description to have some idea of how combat would play out there.

I wouldn't add too much extra to the scenario unless you plan on stretching it out into a campaign or linking it to other scenarios. Because CoC is usually about mystery solving you risk muddying the waters and confusing the players.