I use a K&B 28 and a 20 for sport flying and it is a worthwile and easy conversion.
Get the Fox 25BB venturi and needle valve assembly and remove the carb and install this unit and your ready to go.
This is not a 4-2-4 type motor and you also need to use a fuel with about 25% oil with Castor in it. I generally use the stock muffler and it is very effective in reducing noise. Power loss on the models I've flown them on has been unnoticable.
Both the 20 and the 28 like a larger prop then you might use an a similar displacement from another manufacturer.They like aload and behave like they are larger then their displacement says they are.
Biggest model using both of the Sportster engines has been the RTF Primary Force from Sig and yes the 20 will fly it through the pattern. The 28 with an 11/4 will really suprise you, the 20, use up to a 10/4.
Fuel economy is extremely good on both of them and I use a 3 oz tank and have no problem getting the pattern  done.Line length varies from 58.5 to 61 feet depending on engine.
Use good fuel and don't overrev them and they should last about a lifetime. Both of the ones that I'm using were bought in the 80's.
Dennis

I used a K&B .28 on a SIG Twister back in the '90s.  I used the stock muffler (quiet), K&B 1L plug, 10x4 Graupner prop and 10% SIG Champion fuel.  Set the engine in a 2 cycle and flew.  I won beginner at SIG in 1993 with that combo.

John Miller convinced me to try the the engine after I saw him fly his K&B .28 powered stunter.

Fred