once re-assembled, use a cheap oil, start up and let it get warm, and add engine flush, leaving it to do its thing for 30 minutes fast idle or whatever the tin instructs.
keep an eye on your levels, and if the rebuild seems to have been a success and the leak cured, drain down and then refill with your usual brand of oil and spec.
you could even repeat the cheap oil and flush if you think the engine was particularly gunked up with emulsified oil/water mix?
flush it the once, drain it, refill with a cheap oil again, do a few steady miles, and then flush and drain again before settling on the good stuff.
away you go!
no need to start flushing through with diesel, paraffin, or other such liquids in my opinion, I put my faith in the product designed for the job and use it regularly.
all the best, good luck