ratio

hello Fellas, i just joined today (already posted this question, in the wrong forum) and hope someone can help me out. im about to start my mini bike project. i have a 5hp B&S motor and will be using a rear wheel that is about 16" diameter. i plan on using a two speed jackshaft but not sure...

if i am going to run 1:2 ratio rockers on a stock cam with valve lash caps and 26 pound springs do i need after market retainers ?