Frankly, it is probably more trouble than it is worth. The source comes with RC5, so you could try compiling the source with Delphi 6. However, I'm not sure if the RC5 source would even compile in Delphi 6 anymore. The biggest issue that I can think off the top of my head is that the XP/Vista/Win7 theme support code is built into Delphi 7 and later. It had to be handled by a separate set of units in RC4 that are not distributed with RC5. There may also be other issues well.
I suppose you could try to take the Calculator code from RC5 and copy it into RC4, but I cannot guarantee that this will work.
You might have better luck upgrading your project to Delphi 7 from Delphi 6 and then you could use RC5. Then you could upgrade to Delphi 2010. The upgrade to Delphi 7 should be pretty straightforward because you do not have to deal with the Unicode string conversion, which will need to be addressed in moving to Delphi 2010.