well thanks to the sticker.. no doubt about it being a Rupp Model K enduro

Puts it in the early 1970s (Model J was 1968... so later than that for sure)
value???
Seen a single rupp turbine front wheel selling for 100
and nearly blank chassis for almost 1k
And if I had to put a pricetag to that thing
it'd be between 1000 and 1250 w/o engines.
assuming you still have the rear axle and wheels..
if you have the seat padding add 250 (1250-1500)
if you do not have an axle or the rear wheels remove 250 (750-1000)
check out lostenduros.com
maybe you can find some usefull info there
and here a Model K Ad for your viewing pleasure:
'sid
Oh look bare chassis (no nothing)
https://www.ebay.com/itm/Rupp-Go-Kart-1968-K-Kart-Frame-Enduro-/254035223341
still asking 325 ..add supports and beams, tanks and front... yeah 750 or more is reasonable IMHO