Any replacements for redundant PLC other than Siemens's S7-400?

first you must determine the performance class for CPU after that you can find the range on other brands please find the link below for more information about the six performance classes

Six Performance Classes

Control Logix PLC or DCS USED AS REPLACEMENT.

S7- could be used if software redundancy applied..

Please download the following manual for more details:

Software redundancy for SIMATIC S7