I'm a past IChO participant and together with other past participants, we annually organise the Baltic Chemistry Competition, which is the Latvian selection to the Mendeleev olympiad, but it is made accessible for anyone. The PARTICIPATION IS FREE. However, you'll need to validate your identity.
In the past, people who have performed well in BCC, have also done well in IChO and Mendeleev. There is no syllabus, but the difficulty of tasks is somewhere from National to Mendeleev level.