What everyone here has said is true. The original engine in the MGB is dang reliable. The only catch is the state of the engine and electrics when you got the car. Once you put everything right it's just as reliable as a new car, and it starts easier/faster. My '76 would literally start just by bumping the ignition key (i.e. the engine started running on the 1st compression cycle). I've never seen a ~NEW~ car start that quickly; every ~new~ car I've been in with all their electronic this and computer controlled that has needed at a minimum 2 compression cycles (more commonly it's 4 cycles though) to get started.
In just my humble opinion, about the only thing you can do to a MGB engine to make it 'more reliable' than it already is, is to switch from a points ignition to a points-less electronic ignition. Even that isn't really making it 'more reliable', it just takes 1 item away from the quarterly PM.
The MGB bottom end is bullet proof. If you want more power to go with the reliability, you can have the cylinder head worked over to give you 25-30% more power with a free flowing exhaust, and STILL have a super reliable engine.