if the stock cylinders arent ported for a 4 mill, its not going to run well. They will either need to be ported or they make spacer plates that mount under the cylinders to correct your port timings. A longrod crank uses different pistons with the wrist pin move up higher. Theres several different monoblock cylinders, most common is called a cub cylinder. Theres a version of cub thats called a serval cub which you might look into for trail use. Stock heads use a gasket, aftermarket ones use o-rings.