You could try using a flexible claw pick up tool. Something like this:



Really cheap here

I've lost bits and pieces in similarly deep, awkward spaces and used a flexible grab with good success. You will have to be patient, fish around for a while, maybe take a break and come back to it, but you will get it in the end.

Drain the oil first so you know roughly where it'll be (in the bottom near the drain hole). You never know, there's a slim chance that the broken bit maybe short enough to find its way out with the old oil?

Mind you, if you're going to drain the oil, just unsrew the filter gauze assembly from the bottom of the down tube, remove it and the pipe to the engine. That hole should easily be wide enough for it to fall out.

__________________