I'm looking through some old literature for clutch info.
Might be a Hegar "Mystery Clutch".
I'm pretty sure you have an old sidewinder Enduro kart.
The front of the chassis is pretty basic so I'd guess (but it's only a guess) Late 60s to early 70s.
Go to the Hartman link I gave you, click on the Meyers Kart page and look at the pic of the Meyers Lancer, it'll show the motor mount you have attached.
Your kart has the mounts on the frame for the saddle tanks and looks like plates for front brakes (Sprint Karts didn't use them).
Your latest batch of pics make me think the steering upright is original.
I just can't say what kind.
I don't think it's a Hartman but I'm not an Enduro guy.
Check the sites I posted, you'll probably get an answer.