I have used K&B .61 engines for years as I have several of them. Even though they are an older design, they have always been my favorite 2-stroke .61. They are not much bigger than a .45, are very light and put out decent power. They are extremely reliable and durable as they are double ball bearing, have a steel chrome plated cylinder with an aluminum ringed piston. They are still made by MECOA but you can buy an excellent used one for $60-$70. The are not a high RPM engine as you need to keep the RPM at around 12,000 or less for long life. They have a pressed in crankpin and continuous RPM at 13,000 or more can cause the pin to fail (I have done it). I run 11x8, 11x9 and 12x7 APC props (about 12,000 RPM) and the torque is there for great performance.
They are also very good on fuel consumption.

Good luck with you decision.