i have found a merlin rocket for sale locally.i am assured that evderything is ok with the boat other than it has has some work done on the bottom. work has been done on the bottom but necesitates a hole being cut to allow a holw for the centre board. is it safe to buy this boat and how dificult is it to make good.

Hole for centreboard does that mean a new keel and the need to cut a slot? If si not with a sanitisded barge pole unless you are Garry ruckildge!

yes it does mean a new keel and need to cut a slot. how do i do it or where do i get it done?

Unless you are a very good & experienced woodworker and have a router too get someone to do it for you any local boatbuilder or even carpenter should be able to do it.