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As Reza notes, the 3945 could be used as a replacement for the 3745. However, current gen ISRs are two generations newer (and faster) than the 37xx series. You might find the 3925 well capable of replacing your 3745, as might a 29xx.
A 3745 is rated at 225 to 250 Kpps. Even the 2901 is rated at 327 Kpps.
