Hi Gaurav,
P89C51RD2 or P89C51X2BN is euivalent controller with P89C58X2BN & i is easily available in the market. The only difference is flash memory size,Kindly check with both data sheets for pin features.If your code size is less than 4KB, you can use P89C51RD2 or P89C51X2BN controller for one to one replacement. It supports keil but change the header in your coding & chip select.