The museum is a small but neat building with a lot of interpretive signs, in English and Tok Pisin.
If it happens to be closed the keys are available at the hospital.
Not really "Plan B" but another alternate way to get to Kokoda. Used by many people is the Kokoda Trail/Track. Not recommended to do it on your own, you really need a guide. Anywhere between 6 and 10 days walking from Owers Corner. Be prepared for some hard slog and tricky water crossings.
An alternate way to get to Kokoda is to take the Airlines PNG flight to Popondetta and then get a PMV to Kokoda. There's daily scheduled flights from Monday to Saturday (not Sundays) between Port Moresby and Popondetta. It's only a half hour by plane but three and a half hours for the PMV section. (Check Air Nuigini as they also have flights between Port Moresby and Popondetta.)
Most of the flights between Port Moresby and Kokoda are charter, but there are scheduled flights Tuesdays and Thursdays. The Airlines PNG plane comes in the morning to Kokoda and does a quick turn around back to Port Moresby. Flying time is only a half hour.