That is definitely not a Dumas boat. The cockpit and engine cover say it's a 1/8 scale model of the 1987 Miss 7 Eleven which was driven in full size by Steve Reynolds. The boat looks to be built from Roger Newton plans just due to the design. Now, to answer the rest of your questions, I'll take them in order:
1) the engine needs to be a marine .67, smaller won't push it and larger will over power the hull and make it unstable
2) the 1/4 tube will work for a 3/16(.187) flex cable but it will be a bit tight so lubing will be mandatory
3) finishing works best before attaching the tails and rear wing as it's difficult to tape off and paint around them
4) to finish the boat, remove the rest of the old paint and then give the boat a thin coat of epoxy. After the epoxy has cured, sand it smooth without going through it, this will be your base. Prime and paint with automotive paint and clear over the top. If you want to give it a scale paint job, you will need to get a paint sheet showing the paint scheme as well as obtain a decal set. Decals are sealed under the clear coat
That should get you started