I am looking to buy a b & s 12.5 v twin motor for a deutz allis 613 special in good running condition

Hi,
You will probably be hard pressed to find an engine like that as the one you have is under-rated. That is that the horsepower decal "lies;" it is in fact a 16 HP engine with a decal that says it is less. Nothing illegal there; only illegal if they say it is more than it is!
Think of it as "good" advertising when you think that it is an awful strong 12.5 HP. I have seen 14 HP 16's and 9 HP 12's in the past.
You have a 40 cu. in. opposed twin. A V-twin would be a good replacement choice as it would be OHV, use less fuel per HP and cu. in.
One word of warning, be sure to know the diameter of your crank. There are two common ones, 1" and 1 1/8" and they are of course not interchangeable.
Peace,
Clarke