Are there small electric saws that can cut beef marrow bones?
A manual butcher's saw will work fine on bone or frozen meat; they cut pretty quickly. (In particular, they will cut much more efficiently than a serrated knife or hacksaw, neither of which has appropriate teeth. Never ever attempt to cut bone with a serrated knife.) Butchers will often use a manual saw instead of a bandsaw for tricky angles or just to avoid having to transfer back and forth between a butchers block and the bandsaw.
The problem with using a small electric saw is that all of them are meant to be held two-handed with all body parts well away from the blade, meaning you'd have to clamp the meat in some way. Nevertheless, if you found a way to clamp things, an electric reciprocating saw with a fine-toothed blade would work fine.