Just a wild ass guess hera, but I'm thinking a throttle assembly off a motorcycle would be the way to go. It's gonna require a few pieces be fabricated fo' sho'.

You could use the clutch lever assembly for a squeeze type, but that would take some more tinkering to get it to open the throttle all the way.

Have a Lemmy Kilminster kind o' day

You know I'm going to lose
And gambling's for fools
But that's the way I like it, baby
I don't want to live forever

(And don't forget the joker)