The best pump to replace the Rena Flow 400 is the Rena OEM Arteich 400 which can also be laid on its side.
Depending on the cord you have you would choose 1 of 3 models:
RF400S = RO400S 3 wire grounded cord
RF400C = RO400C 2 wire indoor cord
RF400-LV = RO400LV Low-Voltage with transformer
All three of these pumps have a simple built-in flow control should you need to adjust the flow out of the pump.