The b43g is an 18 hp 43 cubic inch displacement engine with a points ignition system. The p218g is an 18 hp 48 cubic inch displacement engine with an electronic ignition. The p series drive shaft is 2 inches longer than the b series drive shaft. Additionally most b series engines are bendix style starters while the p series had solenoid shift style starters. On the b series engines the bendix style starter mounts to the oil pan and the p series mounts underneath the number one cylinder. The deal with the starters is the solenoid shift starter requires a hole in the engine mounting plate.... Which is easy enough to do but worth noting. There is some overlap is stare styles with some b series engines having the solenoid shift starter attached underneath the cylinder.
It may be easiest to swap out oil pans....... Less electrical work by not having to move the solenoid from the tower.