You didn't mention what vehicle it's for. The stock mounts I am familiar with are Neoprene rubber and not any type of Urethane.
Energy Suspension makes polyurethane inserts designed to work with OEM mounts. They work well, I have their insert kit in the front tranny mount on mine - fit well. But, since they are designed to fit the OEM mounts as an "Insert" to basically fill the air gaps in the OEM mounts, the OEM mounts need to be in some kind of condition to accept them and fit correctly. If the stock mounts are in too bad shape, then a "fill with goop" approach is probably the way to go short of full replacement.
Assuming the stock mounts are salveagable, installing a set isn't too bad. The rear mount can be some work. Really depends upon skill level and what you have for tooling. Long pry bars, hydraulic floor jack is pretty much needed. A cherry picker and nylon web straps are real nice to have on hand for such a feat. That's what I used as I did a complete engine swap.
http://www.summitracing.com/parts/ENS-16-1103R
The original mounts in my 1991 hatchback were pretty wasted, especially the front tranny mount. The ES inserts are made to fit OEM mounts, so I went to the Honda dealer and ordered a new front tranny mount and on eBay and ordered the Energy Suspension insert kit for it.
For the other 2 tranny mounts and the LH engine mount, I found a good deal on a set of Drop Engineering mounts for $120 on eBay so I bought them.
The Drop Engineering mount set is a 3 mount kit and they don't make anyhing for the front, that is why I went with a new OEM stock mount, enhanced with the Energy Suspension insert kit.