Engine weight to total vehicle weight ratio?

[Big time]

What relatively modern cars (60s-on) top the list?

Is there a car or racecar with its engine accounting for half its weight?

[Matra et Alpine]

Quite often for VERY low weight single seaters with compsite chassis.
Check out any hillclimb special
and do you mean JUST the engine or engine and tranny ?

[csl177]

Aside from Matra's suggestion, only 60s jet-powered LSR and drag cars come to mind. Definately more than half.
Some aero-engined specials and sports racers come close, but probably not 50% of total mass.