OHV systems have more moving parts for the valvetrain, which does not allow high rpm easily, so pushrods are more often found in low rpm, high torque engines. OHC and DOHC have less moving valvetrain parts, which allows for higher rpm, so over head cams are often found in high rpm, high hp engines. A pushrod engine can be made to rev high, but it takes more work because there are more valvetrain parts to lighten and strengthen. A overhead cam engine can be made for all low rpm torque, but then it might as well have pushrods because it is not putting its lighter valvetrain to good use. Overhead cams also allow better airflow into the head. I know there are some current japanese motorcycles that have heads that flow so good, they do not need any valve overlap at all. This is why they can redline at 15,000rpm and still have a smooth idle. An extreme pushrod engine designed for high rpm hp will have so much valve overlap it will barely idle.