Missing population values in census data
I think you need to be wary of using curves to extrapolate beyond the age thresholds - specifically I think you should consider:


mortality increases with age; I would imagine it increases at an increasing rate with age (especially at higher ages). Would you be able to capture this effectively?
there is obviously some sort of overlap between populations in subsequent years (e.g. someone who is included in 1985 data could also be in 1986 data). What effect would this have on the data imputation?


I would suggest a different approach. Actuaries have traditionally produced "life tables" which capture mortality in population cohorts. If you can find a set of tables which is applicable to the period in question, you could use these to calculate population numbers.