Well to convert to a needle bearing rod, you'll have to make a all steel rod so the rod can serve as the outer race and the crankpin becomes the inner race. The steel rod allows you to keep the overall size of the rod down so it can fit in the crankcase. Now on many engines with bushed rods, they don't harden the surface of the crankpin all that much, so it may or may not last a long time. You would want to use a caged needle bearing setup too. Using a steel rod allows you to make it more thin so it has less drag when the engine is running.
Some model engine companies did this way back in the 1960's Like Fox for example on their .60 and .74 engines at the time. But it was more expensive though, so eventually they quit doing it. As glow fuel with 16% to 20% oil content worked fine with the engines and bushed rods.
