Minto,
If the bike is running well, there is a very good chance that apart from a set of gaskets and maybe some new tab washers here and there, that will be all you need. The previous posts show what happens with the big but.
The BIG BUT goes " But while I'm here I may as well replace...... At worst the crank bearings will be worn, the bores worn and the valves burned. The maggy will be full of oil and the clutch shagged. So it depends if you just want peace of mind or a full house resto.
I would ride it gently as is, in the short term and don't worry too much. To get the crank out is a complete strip.
If you decide to have a go, part strip the engine in the frame, rocker box, head, barrels off. Clutch and Primary drive all off. Timing covers, dynamo, maggy, all off. Then release the engine mounts, Engine and Box come out as one.. lift and wriggle. Split the gearbox from the engine unit. All easy in theory. Most fixings are all Cycle Thread, threads into alloy are Whitworth.
The oil pump is driven by a worm on the crank, which has a locking nut and tab washer, all are left hand thread. Draw off the pump, unscrewing the worm at the same time.
The toothed pinion gear exposed now has to be drawn off the crankshaft, using a thin jawed puller. Do not drive the crank end through the pinion with a big hammer, the crank end is relatively soft and will bend if roughly treated.
Then its just a matter of removing all the retaining nuts and studs holding the cases, warm them with a heat gun if stuck with jointing, easing them apart. I lay the cases timing side down, crank nose through the gap on the trusty Workmate.
Lift off the drive side case, then carefully turn the crank and timing side case over, crank drive splines through the gap then lift the case off the crank squarely to avoid any damage to the timing bush.
The camshaft can stay where it is, as can the conrods. Support the pistons and rods to prevent any sideways load on the big ends. If the engine runs Ok, with no sign of big end knock, sideways slop is acceptable, but there should be no lift on the bearings. Up to 2 Thou is the wear limit.
So, a set of old BSF/Whitworth spanners, maybe a Haynes, plus Draganfly's excellent exploded parts diagrams should get you on the way, plus Roy Bacon's Twins book if you can get one, and all the online how to videos. BSA Service Sheet 206 covers engine dismantling.
Then there is the gearbox to consider.....
Swarfy.