my 1984 B&S on my monkey ward dies when the pto is engaged?

runs fine till trying to engage blades with pto lever, then dies. if there is a safety switch how do I bypass it or is there a clutch safety switch also? and how do I bypass it to check for the problem? thanks, this is an 18hp briggs and stratten model 422700 on a montgomery ward model tmo-33551a. its old but used to run well and I am too poor to buy another or have someone fix it-please help