Sounds like maybe you need a new starter.
But before that....
Make sure the bat. cables are adequate size, and the ends are clean. NO battery ends that simply clamp the cable! Only large gauge cables (as large as you can afford if yours are real small or tired) with molded on lead clamps. Connect the ground wire as close to the starter as possible. Check all connections for corrosion and tightness. Install the largest battery possible.
I got a good deal on a Skid loader once with a 'worn out' Kubota 3 cylinder engine. The only thing 'worn out' was the electrical system. Small inadequate cables with shoddy end clamps, poor ground, etc. I bought the largest gauge cables I could get, and a big battery with 850 cold cranking amps. Then I had a new engine! Or so it then started like one.....and never fails to light off quickly to this day.