Hi David,
Why are you in need of a new motor and ESC if I may ask?
Is the stock propshaft and prop still present, if so what size is the prop (Ø)?
The hull is capable of handling 20 - 25 mph, but will need some driving skills to stay upright when turning at speed.
A standard 540 should work nicely, if you want more speed, go for the Graupner Speed series, as an example, the 3302:
http://www.graupner.de/en/products/5c5b118e-afc6-4ee2-b03a-c3f2e91947c1/3302/product.aspxOn 8,4V the motor will give you close to 10.000 rpm under load.
If this proves too fast for your lad, get a smaller prop to reduce the top speed, or run the boat on 2S.
You'll need an ESC that has a LVC for Lipo's; the Speed 600 I mentioned will pull up to 40A in spikes (getting out of the hole and accelerating out of a turn), so the ESC will have to be able to handle that.
Keep in mind that running on lipo's usually provides more 'oomph' and runtime, so overheating could be an issue.
As most Speed motors have an internal fan, it's important that the cooling slots at the shaft side aren't blocked, airflow through the armature is crucial.
Also, brushed motors need to be broken in by running them under water (half submerges if the motor has a ballbearing on one side) on 3V or less until the water turns dark(ish), stop and check if the brushes have taken on the curve of the commuter, if you're satisfied, dry out the motor (compressed air works well) and put a tiny(!) drop of oil on the rear bearing and you're good to go.
If you're going to use high capacity lipo's and the runtimes get really long, some form of watercooling for both motor (brushtab cooling) and ESC would be nice to keep the heat at bay.
As I'm not familiar with which ESC's are available to you, others will have to chime in.
Regards, Jan.