I've had a mild mitral valve prolapse with no regurgitation, what will be the effect on my heart if im having tooth decay?

None at all: Tooth decay can affect the heart if allowed to abscess and that swelling breaks into the muscle-fascia plane that communicates with the mediastinum (sac that contains the heart). It was reported historically and mostly debunked now that having dental work would inject bacteria into the bloodstream and infect the valves. It is understood now that oral bacteria do not behave in that manner.

Like any infection.: Unbeknownst to many, the mouth is part of the human body. Decay is an infection in the mouth to which your body may build an immune response. Having the body constantly "fight" any longstanding chronic infection may cause its resourses to be "deployed" away from where it might be needed the most. If someone had a gangrene arm, do you think that would effect the heart?

Why risk it: Leaving tooth decay untreated is a bad idea, get it taken care of. Period. But to answer your question, theoretically bacterial from your mouth can travel through the bloodstream and lodge itself on your heart valves. This is caused endocarditis. Having a few small cavities for a short time is probably no big deal, but having severe decay or periodontitis increases your risk for heart disease.