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.