There are a couple variations of drives still available, but probably nothing that'll be a bolt-on.
When Manco was building trikes, they used whatever was available at a certain price. Be ready to cut/weld, to modify anything else into position.
Reverse isn't the end-all-be-all for a lightweight machine. It's usually easy enough to just push/lift it to where you want it. A 600lb, 4x4 ATV needs reverse, trikes don't, unless someone is actually using them to back trailers up.
The story of three wheels and a man...