Everything depends on the weight of the plane achieved. The take off weight of my model was nearly 50 kg and 210 ccm seemed to be a minimum size, I regretted I didn’t use 280 ccm or bigger. If you manage to make the model lighter, you can use smaller engine. In my opinion 160 ccm is probably OK for up to 35 kg.
As a rule of thumb I’d say, take the size of the engine and multiply it by 0.2, and you will get the estimated weight of the model it can power.
210 ccm x 0.2 = 42 kg
160 x 0.2 = 32 kg
120 x 0.2 = 24 kg