500 Twin (Tomahawk) heads and gearbox wanted

[Burny]

I'm putting together a 1957 (ex factory date) 500 Twin to re-power my 1955 Bullet.  The original 350 mill is in great shape and I want keep it that way till I'm old enough to need a lighter bike.

Can anyone tell me where I might find a pair of 500 Twin cylinder heads to suit, a '56+ gearbox and a matching external oil feed pipe to the rocker boxes?

Even better would be a 57/8 500 Twin engine with gearbox complete, basket case or not, as I am yet to purchase the other kit.