Increase start power / switch batteries in parallel

Hello together, I hope you can help me vllt.. With my mopped model is generally known the problem that the battery is relatively quickly empty and no longer enough to start… Italian just. Original is a 12V 11,2ah battery installed…because of the design of the moped (Benelli TNT) unfortunately no other, larger battery fits in. In a foreign forum I have read now that someone has increased the starting capacity with a battery pack from the model building and it supposedly radios Under the bench there would still be some room for a small battery pack, e.g. https://www.conrad.de/…d-energy-side-by-side-t-buchse-206974.html Would that actually work, that I would use the battery pack parallel to it terminal, so that I would theoretically have 13,6ah to start? And if so, what has to be considered? Thanks already in advance