The Wiseco part number is PS90 for the piston kit. This is the piston for a '90 KX 250, it is the only one that has the correct position for the ring pins. I use '89 KX 250 gaskets, the only difference is the head gasket is slightly thinner.
For '88, there in only one KX250 head gasket listed, #11004-1174.
For '89, there are 2 gaskets listed. The same part # as the '88 is identified as the standard gasket (0.26), while 11004-1197 is the thicker 0.46 gasket. If you request an '89 gasket, make sure you get the thin one and not the optional thick one.
FWIW, on the '90 and '91 KX250, the head gaskets are different part numbers than the '88-89 KX250, and the thick gasket is standard while the thin is optional. The thick '91 KX250 gasket is the same part # as the standard KDX250 gasket, 11004-1215. The thin gasket for the '90-91 KX250 is 11004-1217, which should also work on the KDX250.