I agree with the .70 size four cycle on a Cub of this size. Your biggest problem is that OS no longer makes the .70 Surpass, but only the .70FL. I've not had first hand experience with the .70FL but it has gotten a bad reputation as a 'cheap' four cycle with some of the guys I fly with. You might want to look at one first hand or talk with someone who owns one before you buy the FL. The rest of the Surpass series is great if you decide to go with a different size.

Edited to add:
I just noticed you are in the UK. The .70 Surpass is no longer sold in US markets, but may still be available where you live.