Is there a way to check if I need new kingpins in oversize? Do I only need those when the old one is already loose and rotating in the suspension arm?

if a standard pin is an easy push or a light tap with a hammer then either oversize pin or shrink the eye with a weld

or use a standard pin but before fitting it drill a hole in the side of the lug that it passes through and tap a thread for a 8mm bolt too lock the pin in place

Do a proper job, oversize or replace the arm.

or shrink the eye its been standard practice since they started making stuff out of metal

ive got a mate who actually drills and WELDS the pins in - he has won a good few races in the past . that really is a one way street

i try