My two have great fun in their 2.5m SIB, the 4.5 johnson was just the right power, but a bit heavy.
I'm currently sorting out the much later 4HP Mercury I picked up recently.
As a comparison it weighs 19.8kgs with no fuel compared to 25kg for the old Johnson. It can also be uprated to 5HP with a few tweaks to the carb.
Both engines tick all the boxes, as they have built in tanks, F-N-R gears and a twist grip throttle. I will however add a kill switch to the merc as I did to the Johnson.
Over the winter I've dumped the wooden floor boards and found an airdeck to go in it(£10 ebay), plus an inflatable keel(£1.80 ebay) to go in under the airdeck, I'll try it with and without the keel to see whats best. Its going to be the most modified sub 3M SIB around.
Even I enjoy pottering about in it, its great for days on the beach as once the RIB is anchored we blow it up, drop the engine on and use it as a taxi.
The kids also use it all day to run along the beach using a minimal amount of fuel.
I took a little flack last summer when I posted the pictures of them using it, some people think kids shouldn't be able to enjoy themselves, but they are both sensible lads at 9 and 11 and I think it is great experiance for them.
They need to learn the same way I did at their age.
The rules are of course lifejackets at all times, always more than one in the boat, and nobody jumps out at the beach until the engine is stopped and the helmsman shouts "stopped".
To comment on Sprayblonds post, we took the engine off one day and mucked about with it in the surf making sure it got turned over once or twice whilst I was there. It was a game to swim up underneath it. Although not intended as training for a mishap, it turned out that way.
The Johnson will be for sale by the way once the Merc is fully sorted and I'm happy with it.
RIBBED For extra pleasure.
Member of the ebay Blue RIB cover club.
Member of the Bombard 380 Aerotec club