2Star .60 & (I think) .61. I personally bolted my 2Star .60bb in place of a ST .60. I think Pete Ferguson swapped back/forth between a ST .60 and a 2Star .61 Lite in a series of Strega ARFs. I
really like my .60. The 2Stars are fairly expensive, unless you can find a deal through Roy Trantham or friend.
I do have two ST .60's available. One is a light case T&L, NIB. The other is a low time "K" model Big Jim Greenaway engine. At one time, I wanted to compare them...not so much anymore. I'll take $225 for the T&L and $250 for the Big Jim...plus postage and insurance. The T&L comes with a new Scott Dinger muffler and the adapter. The Big Jim comes with a tongue muffler...I think it's a Windy U. unit, and nice, as tongue mufflers go.
The Aviastar .60 is said to be based on a .46 case, so is not so likely to swap into ST .60 mounts easily. Probably would bolt in where a Magnum .40/.53 does...which is about the same as a G.51 in width but shorter fore & aft hole spacing.

Steve