Yes, and no it isn't worth it (in my opinion).
It's expensive, but just shell out the cash for the kelpro replacement mounts. I assume they make them for the u13, because they do for the u12.
You won't regret it. Suddenly you'll realise what was causing all those annoying vibration sounds in your car.
I reckon replace them all.
If you fill the mounts with some kind of rubbery stuff, the following can happen:
1) the stuff will break apart after a relatively short period (ie sikaflex)
2) the stuff will not let the mount move enough to inhibit the transference of vibration to the chassis (ie devcon)
I know some people have success, but I didn't. Devcon would probably have been OK for the rock solid front-left mount (looking at the bonnet from the front of my u12), but the other mounts require some freedom of movement. Devcon is seriously heavy duty stuff. It will last, but not give adequate movement in those other mounts.
If you want more info, try searching more broadly, this has been discussed a fair bit.
EDIT: I just noticed you mentioned polyethylene. I know nothing about that. I think devcon is polyurethane. Not sure about sikaflex.
Topic: Reconditioning Engine Mounts (Read 6863 times)