Sometimes testing them in circuit is a bit hit and miss.
I think the only way to do it properley is out of circuit, but thats a pain on these boards because theres about 30 MPSU45s.
And, ive had transistors that test ok but break down under load.

Pinball Resource have these on special at the moment at USD$87.50 but with freight and exchange rate thats likely to be about $200 Aussie. But still having a new known "good" board would be handy to compare and fault find that one.

Be careful withe the MPSU45s because the legs break off easily and they are usually soldered close to the board which makes removing them quite hard.

The 2n3055 burn marks are quite common. Ive got them on several of my boards. Maybe they all should have had heatsinks on them, but then again there are a lot of things on System80s which could have been done a whole lot better. I think they even say that somewhere in the Marvin guides too. Inadequate ground plains, underated transistors used , etc.