Engines dont gradually slow down, if anything they get quicker before they go pop all of a sudden, theres rarely any giveaway before the black smoke
Sounds to me like you dropped a rod, this is usually following catastrophic big end failure which could be anything, make sure you get a picture of the hole in the block for posterity
I always thought that if an engine was on its way out and needed a rebuild of some sort, then there would be reduced compression.
This would result in variation of burn pattern and uneven pressure build up which manifests to lack of power. Could be totally wrong though and TBH its a bit late for Grundy anyway!