> ... rehosting some Ada 83 software from a 1750 processor ...

The hardest part of the rehost will not be converting from
Ada 83 to Ada 95 (which sometimes takes zero effort).

The hardest part will be converting the parts that were
written in machine-dependent Ada. For example: all uses
of predefined types, dependencies on word size,
compiler-dependent libraries and pragmas and attributes,
and hardware dependent operations (like switching
memory banks).

It is unlikely that the cost of rehosting would be cheaper
by artificially restricting the rehost to Ada 83 because
Ada 95 is very similar to Ada 83 with about half of the
silly restrictions removed and a few new features added.
The new keywords, etc., usually take MINUTES to convert
to Ada 95.

Possibly this project followed what we called the Ada
philosophy in the 1980s, and did a full requirements,
path, boundary, and regression test at the bottom of
each package body.

The Requirements Test tests that each requirement works.

The Path Test exercises each path through the code.

The Boundary Test tests each variable at its high and low

The Regression Test test each change to the code, to make
sure that the code did not regress to a prior change level
(that is, to a prior baseline). Every time a customer
service request is signed off, configuration management
checks that it is accompanied by an additional test
to be added to the Regression Test suite.

If not, the first step to a quicker conversion might
be to invent these tests and run them ON THE ORIGINAL
SYSTEM. With these comprehensive tests in existence,
the conversion costs approach zero.