Hi,
The engine is just the same. Same crank, rods,bores, head. The difference is with the cams ,the d2 has "sports" cams and different jets in the carbs. Modern pistons will give 9 to 1 compression with no head machining. Inside the engine is the same polished detail of a Bristol engine. The B2 is a better engine for road use because of the extra torque lower down and does not need to be revved. The gearbox is much the same but some might have had a freewheel 1st gear which can be replaced. The rear end is totally different so of no use for an AC
Tony
10kph