Rene, you will have problems installing an inboard engine in the Viper 5.95 as it doesn't have any longitudinal stringers to mount or build up to form engine mountings. The inside of the hul has full transverse stringers every 50cm and half height stringers every 25 cm. You can see them through the hatch in the front deck. There is a full length semi circle stringer down the centre at the very bottom of the hull which incidentially has the fuel tank sitting on. I think it the fuel tank goes at least 8 inches down through the deck if memory serves me correctly. The hull relies on the deep v shape and strong deck to make the hull rigid which it does very well.
Also the 5.95 was a special build with the console and 30 gal fuel tank moved back some 30 cm to make the steering position nearer the centre of the boat. With an inboard weighing over 300kgs you may end up with all the weight in the stern and upset the handling, which would be a shame as it does handle very well.
The best option may be to go for the Yam 115 4 stroke outboard or something similar from suzuki. the honda wold also be an option, but check the weight by using water drums at the stern with the current 2 stroke Yam before deciding. Regards Pete