stupid little specs

say someone was building a turbo ls/vtec (which i'm not, just an example) that was sleeved and bored .020" over, with forged instead of cast pistons. or any engine that doesn't exist at the honda factory for that matter. how would you determine specs like ring gap, bearing clearances, rod bolt torque, head stud torque and all those other important little things that you'd find in a spec manual for a stock engine when you're building an engine that has no set specs. it's not urgent, and i'm not building anything at all any time soon, it's just a question that's been keeping me up at night and making my poor brain spin.