There are multiple reverse gears available aftermarket.

Yelvington has the most interesting (IMO), a reversing pulley. They were developing one for the earlier model bikes like yours, but don't seem to have produced it. But, I suspect talking to them may get you one. Expensive it is, but very low geared too.
https://www.yelvingtonusa.com/produc...anical-reverse

Everything else is attached to the transmission.

Baker has the next most interesting to me. In part because of the handlebar controls, but also because according to them theirs is a low ratio (more on that later). It's also expensive.
https://bakerdrivetrain.com/collecti...nt=16098572547

And then comes everyone else, and they all sell on ebay.
https://www.ebay.com/bhp/harley-reverse-gear

All those have a really leggy reverse ratio. Pretty darn close to 2nd gear speed. But, with lots of clutch slipping, it's controllable.

I picked up a never installed Mototrike reverse off ebay for about $600. While I dislike the gear ratio, I love having a reverse.