unglue wood step

Depends on the glue - most adhesives used in boat construction, even inside are permanent and the bond is stronger than the wood. The screws are usually only there to hold the two pieces together while the adhesive cures.
 
You could hold a piece of packing against it and give it a a swipe with a lump hammer. If it is joined on the oblique then it might just come apart cleanly. It is not unknown for glue to be applied over lacquer and this also would give you a chance.
If the joint has been made properly, then too much force would probably tear off any veneer on the engine box, or crack apart the solid wood.
If it really needs to come to bits, you may need to cut off and re-make the solid timber parts.
 
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