I recommend that you refrain from hot powder spraying on complex shapes such an engine block.  Temperature differentials will often cause existing cracks to elongate and new cracks to form.  In my experience cold powder spray deposits were not homogenous enough to give good tensile strength and water tightness.

If you are drilling holes for inspection purposes, you are just looking for induced trouble.  If you need to check for additional cracks, you should just consider blanking off the openings with steel plates and rubber hole plugs and hydro testing with soapy water at 30 PSIG.  If it is a sleeved engine block, this is less practical, but Eddy Crrent crack detection with a small probe is sometimes usefull.  If you must drill such large holes, consider using a core drill,and using the core as a filler plug.

If you are set on hot powder spraying, and  have an oven that can raise the whole block to over 1600 degrees, weld it in the flames at temperature, and cool it over several days, such an approach is more practical.   However,if you have the oven , you can gas weld it with cast iron/ cast iron welding rod or braze weld it with cast iron brazing rod.  these processes are a lot cheaper than powder spraying.

In my experience, you should electric weld cast iron with  99% Nickel electrode using just under 200 degrees F. localized preheat, and maximum 1 inch long passes that you gently pein with the ball of a 4 or 6 ounce ball pein hammer immediately after each pass, (before chipping and brushing off the slag).

Joe Kane