Bernard,
The only caveat that I would offer is that certain brands of BCRs may be larger or differently-shaped than the 8.4 volt (9 volt battery footprint) of the battery in the engine's tender. Before purchasing the BCR, it might be worthwhile to remove the tender's shell, and eyeball where the battery is and what may need to be done to accommodate a differently-shaped BCR.
Regardless, make sure to remove and replace the existing battery with a new battery or a BCR before attempting to operate the engine.