Assuming pistons and cam are installed.
For cleaning the mating decks and heads, careful use of a carbide scraper.
If there is stubborn gasket residue, might try WD-40 and a scotch brite pad.
Always care not to cause damage to the gasket surfaces.
Use link free cleaning cloth and brake clean or acetone.
If the cylinders have had any machine work done, flush out completely with a solvent (IE kerosene) then with acetone or brake clean and coat with a oil (light weight engine oil, WD-40 etc.) to prevent rusting prior to assembly.