Don't give up be patient, I'm sure someone is bound to read your plea for help. And you will be offered help with your repair of your grandson's Model. I would offer you my hand but I'm here in the U.S., Just be patient I'm sure you'll get a reply!
Is it one of the type that's showing on eBay for £90. Or a kit. If it's the eBay type you could end up with a very expensive "toy" for such a young lad by the time you put modelers components in. Unfortunately built lifeboat models don't leave thier owners very often, unlike say cabin. Cruisers and torpedoed boats. If you are a modeller apologies for " teaching your granny etc). Built my son a Robbe Dolly when he was around 10. That was over 20yrs ago. U now have around 24 serious models and either they're getting heavier or I'm getting older. Is there any clubs in your area you can visit and have a chat. I'm inNortheast.
It's a ready built one marketed under the auspices of the RNLI itself. I bought it from a shop called Nauticalia. There are numerous stories about it on the www - overheting, sizing up motors etc. I was offered a refund by the shop but they had no replacements and the little man was/is really attached to it - even though now all he can do is float it in the bath. There are also stories of it being modified to full RC specs, but I don't want to go the pro route, just something he can use and have fun with as originally intended but with better motors (water-proof). I was an engineer and I've modelled locomotives, mended clocks and built 1:1 racing motorcycles - not boats in any scale though. Trouble now is I can't hold anything steady, problems even to write. Despite being on the coast I know of only one model boat club - it's a good one but it's racing 'yachts' only and very pro too.