i machine old screw-in feild points down to fit the ID of the shaft.
once the target weight is reached i locktite red the old point into the rear of the insert with a normal point in place in the front of the insert as a 'gauge' to prevent not being able to install points, normally, once insert is installed into shaft.
if the insert is not threaded all the way thru, simply purchase the appropiate tap and thread it the rest of the way thru
if you don't have access to a lathe, you may use a drill/drill press and file. it'll take longer, but it can be done.