I have the stubby key version of your HM-901 button melodica. Many Hohner melodicas are easy to take apart and reseal, however, the stubby-key and button models are quite tricky to take apart and reseal in my opinion.
I believe most components in melodicas are resistant to water by design because melodicas get wet inside all the time. Water leaks out of my old melodicas all over the place.
I had a very smelly HM-26 and took a chance. I submerged it in vinegar and water and/or a bit of dish soap for about 30 minutes to an hour. Then I rinsed it by submerging it several times in clean water. I shook it all over the place at several angles and put it in the sun on a perfect drying day to dry. The melodica was dry in a few hours and the smell was gone.
I also took a stubby key piano 26 apart (that is how I know they don’t come apart and reseal easily) and threw the individual parts in the dishwasher with dishwasher soap. The parts came out really clean and nothing was damaged. It seems the rubber gasket adhesive is waterproof as you would expect. I did not leave the parts in for the whole wash cycle. I took them out after an hour and rinsed them off. Every ten minutes I rotated them during the wash cycle
Both the above adventures are documented in a couple of my posts. Search on relevant terms and you might find the posts.
Caution: It worked for me under my conditions and with my specific models of melodicas. Your results could vary.
I always ask sellers now if the melodica smells and if it leaks before bidding on a used melodica. I love it when they come in smelling like perfume. Then I know it sat in a dresser drawer for 50 years and was stored properly.
Welcome to the forum.