I have never used MarineTex epoxy for any repair, so I cannot comment specifically about what screw fasteners would work best in that particular epoxy. However, in general, an epoxy resin with some thickening agent (which I presume is what MarineTex epoxy actually is), can cure to a rather hard material. If you try to use a self-tapping screw to cut threads into a fully cured epoxy resin with some filler material, the ease of doing that is going to depend on the ratio of filler to the epoxy resin.
If the mixture of resin and filler contains a very high proportion of filler, then cutting thread into the cured material a self-tapping fastener may be possible.
If the mixture of resin and filler contains a very high proportion of epoxy resin, then cutting thread into the cured material a self-tapping fastener may not be very easy and could risk fracturing the material or the laminate surrounding it.
Generally my experience with epoxy involves using WEST System epoxy resin and filler material. I have found that using self-tapping screws is possible. A pilot hole is a must, and the size of the hole has to be carefully chosen so the threads being cut are not trying to remove a lot of material that will be occupied by the center of the screw fastener.
I generally use one sacrificial screw to pre-thread all the holes. Then I discard that screw and use a fresh self-tapping screw to thread into the pre-threaded hole. The sacrificial self-tapping screw cutting the threads will become very warm after a few holes, as there will be a lot of friction and head created. After that screw has cut threads in many holes, I figure it is worn out. Also, in some cases you may have to back out the thread-cutting self-tapping screw and clear the hole of debris in order to get a good clean cut.
It is also possible to drill and tap a material of cured epoxy resin, and the use a machine screw as the fastener.
As for the material of the screw fasteners, it should be stainless steel and marine grade, not cheap Chinese hardware store stainless steel.
As for the size and pitch, probably at least an #8-size screw and the Fine pitch is appropriate. You can judge this for yourself, and perhaps a #10-screw would be a possibility. Use the existing screws that you removed as a basis. Using 1/4-inch-diameter screw fasteners seems to be to be rather large.
A more important factor is the depth of new material that has been created, and how far into that material the screw fastener will penetrate. Again, the existing fasteners are a good source for gauging the depth.
Regarding MarineTex, I have never understood how that epoxy uses such a high ratio of resin to hardener. In WEST System epoxy the resin-to-harender ratio is typically 5:1.
A good source of WEST System resin, hardener, and filler materials is their 105-K Fiberglass Boat Kit product (which now costs about $60). See:
https://defender.com/en_us/west-system- ... repair-kitIf you have any questions about using WEST System Epoxy, you can contact them for advice on a specific use. I just spoke with them, and for creating a strong base for the screw fastener you need to use their HIGH DENSITY filler, and a proper Pilot Hole is also needed.