I have been involved with karts for many years. Your Hornet Enduro kart would be very expensive to restore ($1,000+). And, there are few places to run or race it. It has a few salable parts (Spindles, tank, etc.)
Look at a local track for a Kid Kart with a Comer engine. Older used ones are cheap and would fit a tall 5 yr. old, be cheap to run, and you would have many places (Tracks) to run it.
Or, the vintage kart above would be a lot of fun, but not as many as many opportunities to drive it.
In 1970 I drove a C-Open (270cc alky) Hornet Allegro Shark. If I still had it I would have no interest in restoring or racing it again.