Need new Diesel Engine for Saber Tooth Tiger

i have a Scag Sabre Tooth Tiger in good condition. My engine warms up and the oil pressure drops. I am guessing that I need a new engine. Any recommendations on buying a new engine or getting this one overhauled?

Love this mower. A hired hand used to overheat by not cleaning the screen. Also, did not do a cool down. The turbo is probably in need of replacement