Hi Skippy,If cranking speed is slow/laboured (even with a new starter) try improving the (-) ground /earth return by using a jump cable between (-) battery ground/earth and anywhere you can get a good connection on the engine.
!2 Volt systems are notorious for developing (corrosion/loose connection) a poor (-) ground/earth return. Clean ALL your earth return connections, making sure they are tight.
I do not rely on the earth return through chassis/body and install a dedicated earth return on all my own machines, running from battery (-) to any suitable point on the engine (usually a engine securing bolt) and rarely have electrical problems.
All connections were cleaned, scraped and even sanded (where appropriate)…
Thanks,
David