RE: Vintage K&B 61 power RPM
The larger prop results in more efficiency so that will make for a improvement.
Based on the numbers you provided, the .46AX engine develops about 1.5hp and the K&B .61 develops about 1.4 hp. The speed difference is under ideal conditions the .46AX might go 85mph whereas the .61 might go about 81 mph.
So you may not see a difference between the two engines but the larger prop on the .61 engine may feel like it pulls stronger though. Even though the static thrust is about equal for the two engines and their respective props.