Previously, we noted that an engineering firm can bring related experience to bear on an issue, often producing an elegant solution. But if you specify what you think you want, you might not get that elegant solution.
So begin with a clear and concise explanation of the problem. For example, "This uses a mechanical linkage system that breaks and that we can no longer get spare parts for. We have to fabricate replacement pieces."
You want the engineering firm to look at the equipment and offer solutions for replacing that entire system. Keep your own list of ideas, but don't share it until time to discuss the various options. If you mention a servo system, they may think, "That would do the job and that must be what the customer wants, so that's what we’ll do."
Don't try to save money by doing this part of the job. Tapping the experts will give you the biggest bang for your buck.