It is a little hard to be sure what engine you have. I have never heard of an Irvine 50, as far as I know they made 39,40,46 and 53 size engines that might be in a Tutor

As it is a trainer speed is the last thing you want so I would go with a lower pitch say 4 inch so a 11x4 on a 39 to a 13x4 on a 53.