In the unlikely event that anyone else has this problem or for that matter if anyone is still following this thread the SC24 head can indeed be fitted to a SC21 engine. Just be sure to use the SC24 head gasket as well to get the internal water circulation correct. Everything else is the same.

 

As an update to this thread, she's just had her first service and is loosening up nicely. The power is feeling good and solid. After I finalize the exhaust setup and jetting I will get her on a dyno and let you all know how she did. Still think a little more mileage before the dyno to be 100% sure she's loose. That big bore cost a few bucks and I don't want to run her too hard too soon. 350km on the motor so far.