He is missing !!!
I started with the marriage of John Muncaster and Ruth Hellen ,( From IGI) 25/12/1790 Saint Bees, Cumberland , England. Children :
All the above at St Nicholas, Whitehaven, Cumerland, and possibly at Moresby, Cumberland
John 30/1/1803 Kirk German
Ruth 11/8/1805 Kirk German
Elizabeth 5/7/1807 K German
William 17/6/1809 K Lonan
James 26/11/1815 k German
I looked for a birth on the IOM for Isaac with any surname 1805-1820 and although there are 10 , but none look like the one you want.
James is shown as a death 3/6/1816 by both the IGI and Brian Lawsons burials.
So as William was born Lonan , looked for him and found two marriages , but neither show the father's name.
Marriage of Issac and Elizabeth
Isaac Muncaster of the Parish of German to Elizabeth Wade , a minor with the consent of parents.
They appear to have at least 10 children , including Isaac Gunson ch 17/8/45 who is not with them on 1851 census and is not on te burial list, and very odd for the time, a Scott ch 11/9/1851.
Not much help for what you want, but maybe something you did not know.