The LT 40 usually is powered by a cheaper .40 2 stroke,since it leads a hard life as a trainer.However a four stroke will fly it just as well but is a lot more expensive to replace or repair when the inevitable indiscretion is committed with terra firma,or other beginner faux pas (lean runs,dirt in the intake,electric starter used on an engine loaded with fuel,stripping the plug hole etc...).
The rule of thumb used to be that you needed 50% more displacement when replacing a two stroke with a four.....(IE a .90 to replace a .60)but I doubt this is necessary today with the progress in four stroke designs.four strokes are heavier than equivalent two strokes due to the extra mechanical bits so watch your CG position.A .50 should be fine but as I say check the CG carefully.