I think most people on the forum use 1708 for the engine mounts. I've never worked with woven roving, but from what I understand it's a little more of a pain in the butt to work with around corners, and you have to use layers of CSM in between each WR layer for proper bonding. Also, I think I've heard the strength to weight ratio of 1708 is better than WR all said and done. In WOG's guide, he mentions using 4 layers of 1708 on the engine mounts, rather than the standard 2 layers for stringers and bulkheads. If you follow his suggestions you should be just fine.
In this link, starting at post #213, I begin fabricating my engine mounts and doing the install. They are a slightly different configuration than yours, but may still give you some ideas on how to proceed.
http://forums.iboats.com/boat-resto...90-engine-mount-stringer-repair-636552-9.html
I also did a sister joint on my stingers up near the bow. Take a look at post #111.
http://forums.iboats.com/boat-resto...90-engine-mount-stringer-repair-636552-5.html
One thing I may suggest, if you haven't already, take many many many many many many measurements of your engine mount dimensions and heights, and from various reference points. I didn't see in your thread which engine/outdrive combo you have, but if it's an Alpha One paired with a 5.0L, it will be identical to mine. The forward engine isolators (between the engine block and your fiberglass mounts) that lag into your glassed in mounts have about 1/2" of adjustment up and down, so you'll want to get the final height of your engine mounts as close to possible to spec.
Here's a couple of installation manuals with dimension specs, right from Mercury's website. They only go back to 1996 for some of the drawings, but they should be close.
http://www.mercurymarine.com/media/mercury/drawings/alpha1.pdf
http://www.mercurymarine.com/media/mercury/drawings/50l57l.pdf
And here's a link to the page to search for documents, it's kind of hard to find from the their homepage.
Installation Drawings | Mercury Marine
Looks like you're well on your way, good luck sir!