Richard (and anyone else contemplating this mod)- please see revised schematic below
Functionally, the original is OK - however when start signal is present, the diode feeding start signal to the new latch relay, will also supply current directly to the input of the start relay - that means current to the start relay will actually be shared from the start control signal itself and the input power (Blue/Yellow)
More correctly, the latch feedback originally shown as a jumper should be a diode.
The downside to this is you cannot connect the start signal diode directly across the start relay (between pins 1 & 3) - although there is no real requirement to change the suppression diode location between 85 & 87 - the new additional diode between 87 & 86 will serve both as a froward diode for the latch and and also in reverse for the syuppression, where it will effectively be in series with the suppression diode connected as per original.
