Unfortunately changing a Ketch rig to
Sloop rig requires allot of structural changes.
Remember a sailboat needs to be sailed against the
wind as well as with it.
Firstly the
sloop mast should be moved aft to regain balance, as removing the missen sail moves the centre of effort from the sails forward, which would have to be countered.
Secondly with a larger sloop mast ,you are lifting the sails moment of effort higher, which you would have to counter with increased ballast.
Thirdly, new mast step would need new bulkhead for structural support together with reinforcing new chainplate attachments.
All of these measures really require the efforts of a Qualified
designer or experienced Shipwright and then it is still a gamble whether you would be able to steer it. My best
advice would be to change it back to a ketch, that is what the
boat is designed to be...either that or buy another boat.