They did eventually replace the Buicks.

> The original design was conceived by two Lockheed Skunk Works engineers who had prior race car engine knowledge. In the mid-1970's, the engines of the start carts was changed to Chevrolet LS-7 454's. The Chevrolet big block engines developed 465 horsepower each. Even so, for many years they were still referred to as the Buick's.

https://www.thesr71blackbird.com/Aircraft/Engines/starting-t...

And eventually the start carts themselves got replaced.

> In the '80's the AG-330 Start Carts were mothballed and in there place came a Pneumatic Air starting system for the SR-71's. It is true that the start carts were hard to maneuver around. Logistically, a recovery of a Blackbird at another base could be done easier with Pneumatic Air.