2003 Klein Attitude comp bottom bracket change type
I googled a few pics of the frame - looks like it has a standard 'English' threaded bottom bracket shell - 1.375" x 24 tpi. Cartridge square taper BBs are readily available for this shell.

You'll need to measure the width of the shell, it should be 68 or 73mm. You'll need a BB unit compatible with that width.

The tricky part is figuring out what axle length you need to get the chainline right for your bike with the crank you have. I don't have any direct experience for that but there are at least a few questions and answers on this site on that topic.

You'll also need to check the square taper type of the crank. I think the JIS type is the most common. More info here.