i dont know much about matchless twin history or vallue, but i can tell you that those crankcases are worth big money ... especially as you have the barrels and as near as i can see, the crankshaft. if you have the heads as well, so much the better ... and if you have the genuine racing gearbox you can write your own check. as far as im aware the G45 was a factory GP racer, and matchless only made a few hundred in the mid-fifties
im really not competent to give a price estimate, but dont let anybody swindle you; look in bike mags and things to see if you can get prices for G45 bikes/parts