TBO of preserved Rotax Engine

Assuming a newer engine with a 2000hr/15yr TBO;  The 2000hrs will be Hobbs operating time and the 15-year clock started the day the engine was assembled, not installed.
The seals and O-rings have been aging in the box.
The recommended 5-year rubber parts replacement will be due in 3 years.

Rotax recommends that any engine not put into service within 2 years be overhauled before use, but that will be your call.

Not so sure about that...

Rotax states a general overhaul is required after a defined time or hours "since initial start of operation"

Therefore the TBO starts "since initial start of operation"

It does NOT say from date of manufacture, or date of install.

It does later state that if the engine maximum possible storage is 24 months.

Therefore I read that to mean you can store for up to 24 months then TBO starts from first start.

Although the rubbers etc will be a little older, the main cause of rubber deterioration is heat, and a stored engine does not get "running hot"