BlazingPedals

I don't have any advice for supplemental power, but ... wow! A 12-inch gear isn't low enough???

edit: I just did some recalculations. Catrike lists the gear range for a Trail as 18-90 gear inches. IIRC, the DD has gear ratios of 1:0.73, 1:1, and 1:1.37 (just like its predecessor the 3x7 hub.) That means low gear would be 18 x 0.73, or 13.1 inches. Plus or minus a bit depending on which tire you're running.

To get an 18" low gear, the Trail would use something like a 30T ring in front and a 32T cog in back. Before adding power, which might tangle you up with local definitions of mopeds and/or motorcycles, if you really need a lower gear, then you could consider putting a mountain bike crank set on the trike. That would give you something like a 22/34/44 in front, for a 13.4" gear even without using the DD's low range. Including the low range, you'd have a 9.8: low gear, which is almost too low for me to imagine. With the high range, you'd still have a 106-inch high gear for downhills.

Changing cranksets would likely mean replacing your front derailleur with a MTB-specific one.