Either you can obtain a fresh promissory note with a fresh witness to dispel your suspicions, even without all this the role of witness steps only if the executor pleads that his signature was forged and refused to repay the amount within 3 years from the date of the promissory note. Witness is not a guarantor, and his role and duty are to confirm that the promissory note was signed by executor in his presence.
1. EVEN "IF" the witnesses to the promissory note is dead /untraceable /turned mental, it does not affect the legal status of the Promissory Notice. The liability will legally prevail, in court of law.