Yea, i don't think anyone uses brass anymore. Believe that was the original fix but it didn't last.
The bearings move in the bearing pockets when the crank flexes at high RPM, regardless of the material used to make the sleeves for the bearing pockets With bearing movement in the pockets, ware will occur. None of the insert materials I have tested lasted for ever.
When the bearing pockets and bearings are both made of an iron based alloy, micro welding occurs, accelerating the pitting and deterioration of the interference fit between the pocket and bearing.
I have experimented with a lot of different materials for bearing sleeves and found that the bearing sleeves that Rotax used in some of their problematic engines, also solved the problem in the LT 500 bearing pockets. Rotax has discontinued making these "plastic" sleeves. I have talked to Rotax and they said that the material is proprietary and it is not Delron.
My second choice in all of my experimentation was bronze inserts. The bronze inserts lasted as long as the Rotax plastic sleeves but the Rotax sleeves cost about 15.00 per sleeve where the bronze material cost around 180.00 per foot plus the added cost of machining the sleeves from the bronze bar stock.
Rotax's discontinuation of the "plastic" sleeve has raised the cost of re-sleeving cases due increased material cost and two more close tolerance machining operations in the repair of worn main bearing pockets.