If we use Formula Atlantic motors as a guide you can probably get another 10% when you,only have to make one race weekend on a motor. Part of the increase is revving the motor an additional 600-700 rpm.

Back to the A13 increasing the output by 10% would take the motor to around 180-185 horsepower range. Someone with access to unobtainium parts and support could probably improve on that.

With the previously mentioned sports racer guys I remember here that they were spending $2,000 on custom cranks. That was back in the early 90s. It's amazing what you can do when you have a budget of $15,000 for an engine.