It's worth noting I'm not a lawyer (I advise you to check in with one) but have played a major role in 4 acquisitions.

I would have the contract read that your client pays the success fees, seems a bit odd to have the acquiring company pay the success fees. Let them negotiate with the acquiring company on covering it, but contractually have them responsible.

If the above is not an option make sure the assignment clause in you contract with your clients reads correctly.

That said keep in mind that a lot of acquisitions are not whole business entity acquisitions - meaning in a lot of cases it is really an asset sale. By only purchasing assets the acquiring company can protect itself from litigation and debts. If it is an asset purchase they may be able to get out of paying you - again this is why I would push for the success fees to be paid by your client.