STL file 3Dexport - not scalable

My first attempt - top left
STL of W154 top right
Variety of body shapes W154 bottom left
1939 W154 (incorrectly labeled’W163’) bottom right

Wagen 154 - my first try on left, and Mercedes object of beauty on right

One of my attempts at scaling

Scaling in Onshape - keeping to the Cycle Kart regulated dimensions

Scaled Honda GX200

Scaling in Onshape

Scaling me and the engine

W154

Part scaling W154