You want the engine to be able to tilt - in the tilt position on the lever if it's present. But you should run strong bungees around the leg. Putting them around the transom and leg is an easy way to do it. Should be photos in my thundercat thread on here, means if you hit anything in the water there's some give in the engine to stop breaking things. You want them quite tight, so that when you drop off the throttle suddenly the engine doesn't kick up.
You don't neccessarily need to run a steering damper, if it feels fine without one it probably is. If you need one you would know about it! Generally they're required when running if rough conditions, they help prevent the tiller snatching as the prop bites when re-entering. You can get all sorts of expensive ones, hydraulic are the most pricey and are good for long-haul racing. Friction dampers are cheaper and are used for surf racing, although either kind will do for general use. A cheap option which is favoured even amongst some of the fastest boats is to use a Mercedes steering damper, they're about £18 on ebay. Can't mind the part no. but it's from a 1980's Merc, although really any damper would do.
For the floor you can do it as cheaply or expensive as you like. Marine ply is fine, you'll want to waterproof them with something. I had some custom composite boards made up for mine, then I cut them to size and gel coated the cut sides. Waterlogging is a big problem, you realise it once you feel the weight differential between old and new boards!
They also start to flex a lot more than you want, especially around the corners.
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