The most important thing to keep the Spanish Authorities off your back is to get the boat UK registered, it only needs to be Part 3 Small Ships Register (SSR). This costs £25 for 5 years and you need to put a small set of numbers on your transom and I would also fly a UK red ensign and a Spanish courtesy flag. This will significantly reduce the other hoops you have to jump through. Register online : https://mcanet.mcga.gov.uk/ssr/ssr/
Now you can get a bit sneaky if you want, i'm fairly sure you can avoid most of the red tape (Insurance, ICC etc) by having a slower boat with less than 10hp. I'm guessing the old Evinrude is the same block as a 9.9hp ? If so you could disguise the engine and avoid any further problems. This is illegal and I am just making a point.
My suggestion would be leave everything as it is and take the minimum of safety kit. I would also suggest you take proof of insurance and an ICC if possible, this makes you fully legal. My suggestions for other kit:
Mobile Phone (& emergency numbers) in waterproof case.
Large scale chart of the area (Buy in the UK)
All the other things suggested above are nice to have, but have limited use in the Med. I hardly ever used a plotter, GPS, compass or radio in the med. The coastline is fairly boring, you either go 'up' or 'down', thats your choice. It can all look a bit the same, but I doubt you will be doing huge distances anyway, and you can trace your route on the chart. Very few people monitor the radio, not even marinas.
The one thing that I would suggest you get is a Bimini, you will fry in minutes out on the boat if you don't have any shelter. Also take a cover to keep the sun off the boat when you are not using it, 3 weeks in strong sun could virtually write off the Honda's tubes. Top tip - never take the boat into a bathing area (Inside yellow buoys) and watch out for pot markers and fishing nets.