We have two Jetworks 4mm engines, a 5mm and a 6mm here in Australia. All of them are resleeved 61x cylinders
I've ridden quite a few ski's with different engine combos and have to say that by far, my favourite is the 4mm running total loss. The power is smooth and strong, so its perfect for the surf. Until you have a bit of good power for the surf, you wonder how you ever did without it. The extra power makes it easier to roll and flip on fatter waves.
These engines surprise a lot of people and the price is right.