you might want to start by getting new batteries, if you want by far the best battery for any vehicle especially a diesel then id get these
PC2150 S and PC2150 T - Odyssey batteries
they are a bit pricey but they will out last any other battery and it has way more cranking power then even an optima.
im not 100% sure but i think that battery will fit, if not you may have to get a little bit smaller one. I have one in our sandrail with a semi high compression 4cyl and a small 13lb odyseey will start it and crank it forever (even used it in my toyota pickup for a couple weeks when my optima went out).
good luck