The mounts may look intact but the rubber inserts may be directionally weakened with use and wont dampen the shock and vibration.
i doubt squeezing in the polyurethane preforms into the mounts will help stiffen the rubber in the right directions.
I rebuilt a few worn front and rear mounts with liquid polyurethane 2-part epoxy with high hardness, about tennis shoes soles. The mounts feel stiffer and work solid but the car will vibrate slightly more than the OEM's softer rubber mounts.
Half gallon of polyurethane epoxy sells for about $40, good for may be 10 mounts, but it has shelf life of only 12 months. Since you take out the mounts you may as well pour the epoxy to make sure it fills the correct locations in the mount. I insert foams into places not needed. The epoxy is liquid like water. I use playdo to seal the bottom of the mounts water tight before filling or most of it will leak out.
I have a few rebuilt mounts on hand just in case I need them. The large top left mount (passenger side) seems to carry most of the engine weight with a rubber ball. Not sure how it works so I bought OEM mount. After market top mount did not fit and unsure how long they would last. If you buy AM mounts, check all dimension carefully on receipt to make sure they fit, before removing the old mounts.
-- snip --
I ordered the poly urethane from McMaster Carr:https://www.mcmaster.com/#urethane-casting-compounds/=16q769u
General Purpose Urethane
Shore hardness Hard (80A) hardened in 15 minutes cure time 168 hours max temp 180° small order 25 cubic inch 1 pound part number 8644K11 $31.91
-- snip --https://www.mcmaster.com/#urethane-casting-compounds/=16q769u
Easy-to-Work putty, not liquid, Casting Compounds. Hardness 60A would be about same as rubber. 80A would be like tennis shoe sole.
Combine the liquid components to form a putty that can be molded into or around any shape. Use these compounds to capture intricate details when creating prototypes, molds, and linings.
Work time Cure T Demold Cure T Melt temp Shink % Col Cu Inch Weight Price
Durometer 60A 15 min. 168 hrs. 3 hrs. 75° 180° 1.5% Black 25 1Lbs $31.91
Durometer 85A 15 min. 168 hrs. 3 hrs. 75° 180° 1.5% Black 25 1 Lbs $33.33