If your Honda BF5 is a 4 stroke, it's weight will be exactly the same as the modern one. If this is a 2 stroke, the modern 4 stroke will be about 10-20% heavier.
Regarding the excess of the horse power on your inflatable, it is all about the insurance. Physically speaking, your boat can probably handle an 8 HP but if the boat is rated 6 HP only, any excess will invalidate your insurance. If you don't mind, try a bigger one. My boat is rated 20 HP but I run a 25 HP on it and will update it to 30 HP soon.
A modern 5-6 HP will push your boat with 2 people at about 5 knots, which is below the planning speed, so 5hp or 6hp will not really make big difference.
I was on a 12 ft inflatable with a friend, powered by Honda BF5 and the only times we could get on plane was when the river cruising boats passed behind us, creating a lifting wave
If you want some more power and less weight, the only option is a 2 stroke 6hp, you can get one for not expensive on eBay