If you buy a 5V input SPST solid-state relay with mains-rated contacts, it'd probably work fine straight off the output of your
74175. If the 74175 doesn't have the drive capacity (unlikely) to drive both the new switch & the existing Darlington pair (the symptoms will be that the solenoid, motor, or both fail to turn on when they should), you could easily use a coupleof gates from 7400 or 74LS00 to buffer the output of the 74175. Omron make lots of solid-state relays that'd be suitable for your task. You haven't told us the current rating of the 28V motor, so you'll need to look it up or measure it before selecting a relay, but something like this should do the job:----- Technical/Catalog Information CC1585-ND Standard Package 18 Category Relays Family Solid State Vendor Crydom Co. Type Triac Current-Output - Voltage-Input 5 VDC Switch Function SPST-NO Mounting Through Hole Load Current - On-State Resistance - Load Current @ Voltage 1.5A @ 280V AC Lead Free Status Lead Free RoHS Status RoHS Compliant Other Names LC241R-ND
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Unless you like flushing money down the toilet, don't replace a common part like an MPSU45 with an ECG part unless you're very, very desperate. ;)