Heald Hauler won't go forward

Mini Bike & Go-Kart Parts

I'll admit upfront I'm a newbie and if I'm in the wrong forum - please forgive me and point me in the right direction. I inherited a 1981 Heald Hauler 3 wheeler from my Uncle. I've been using it around my property and yesterday it stopped going forward, reverse works. Thought it was the belt which looked like it was slipping ($40 on a new belt and that wasn't the problem!). I now see that when in forward gear the rear pulley that connects to the transmission just spins (the shaft into the transmission spins also) In reverse it locks in place and away we go. I'm guessing that there is a problem with the transmission - is this something that is serviceable or can be replaced with a newer transmission?
Hello @Andrew E ...welcome to the Oldminibikes.com forum !

You are in the right place. Lot's of Heald fans here.

Matter of fact member @chatten63 now owns the Heald rights, and has a vast amount of knowledge.

One of the experts will be along soon, and can offer some suggestions.

In the meanwhile, any way you can post some pics of the pulley and shaft that keeps spinning when placed in forward.
Glad I'm in the right place! Here is a video of what's going on when I try and go forward - the clutch on the engine side appears to be engaging properly
When I go in reverse I can see the transmission engage, the chain tightens and it powers backwards (and I'm now out of backing room - of course before I shot any video!). The bubbles on the floorboard are from the Dawn I sprayed on the transmission housing to knock off some of the 40 year grease accumulation in order to uncover the stamped part number.
It might just need lubrication and adjustment of the shift linkage so it can shift all the way into the forward position.
It looks like it’s going all the way in, but not engaging. I’m more than willing to start with the basics. The shift linkage I can figure out - where should I try the lube?
I just ran it again now that I’m getting better ideas of what to look at. The shaft on the chain side turns the correct direction when in forward and reverse and is stationary in neutral. When I gas it the sprocket that turns the chain doesn’t move, I’m assuming it engages somehow when the rpms go up.
@Andrew E , thanks for your patience.

@Mariah, thanks for replying.

With the sprocket spinning, perhaps whatever holds the sprocket to the shaft has broken loose ?

C'mon folks...let's get Andrews issue diagnosed for him. Y'all ask some questions, or make some suggestions.

Thanks !
@Mariah, I think you're onto something. I went through an envelope of paperwork my cousin gave me with the Hauler. I thought it was just the book for the B&S engine - turns out it also has the original assembly instructions and every other bit of paperwork my uncle had for the Hauler! The diagram does show a key! That very well could be the problem! I'm going to pull the bed for better access and take a looksie. I'll report back what I find.