I realize the Isaac Allen I know of is not the correct one BUT I think there is a relationship.
The Isaac Allen I know of was living in HHD 8. Washington, Harrison Co., OH (Tippiecanoe P.O.) in the 1870 Census.His age shown as 56...POB OH.
HHD 9 was James W. Allen39b. OH
HHD 12 Reuben Allen87b. MD Catherine A. McIntire. 50b. PA
I believe the Isaac Allen you seek was a brother of Reuben.I believe Reuben (b. ca 1783 MD) was the father of the younger Isaac and James, also Ruth A. who married Leander Bennett and Julianne. Reuben was widowed by 1870.His wife was Joanne.
There was an older Reuben Allen who died in Washington Co. OH in 1819. That Reuben was probably the husband of Magdalene Bird.Father Jackson Allen, mother Betty Davis.
I was seeking information about Reuben Allen to see if he was related to my ancestor, Sarah E. Allen Blair.
The informatio I have is from Censuses.My note taking isn't very good but thought it might give you some clues if you are still at the e-mail address from 2001.Nancy Burke