Will you continue to use the Allis B for mowing after you finish restoring it? If yes, buy a different mower (used, cheap riding mower) and re-sell it in a few months when the B is ready. If no, maybe find another tractor that can use your existing belly mower without too many modifications.
Buying a bigger tractor is a your personal preference. I don't know if an in-direct injection diesel really has much if any advantage over a gas engine for very low-hours of use and stock tractor pulling. You may get better preformance for less cost by staying with an older gas tractor for (near) stock pulling and then move up to a newer direct injection diesel if you move to a higher class in pulling. Turbochagers work better on direct injection diesels.