This one?
https://www.hifishock.org/gallery/electronics/musical-fidelity/integrated-amplifier/a300-1-musical-f...Looks a good DIY to me.
Clean it up, sure - but vacuum and brush, not blast.
What I will now explain is so simple I myself did it years ago back when I knew next to nothing, and it turned out to be the most cost-effective upgrade ever. Nothing beats parts upgrades like this. Nothing. You have to do it to hear for yourself.
The four big black D-battery size things are power supply caps. There's a whole bunch of smaller caps all over the place. Caps have their values written right on them. Make a list. Then go on line and start looking them up. Make another list.
You will quickly find there's lots of choices. Lots of specialty caps. You can go crazy or you can be sensible. Sensible to me means you have some ballpark budget and buy only the caps that fit that budget.
Pay attention though because another thing you will quickly learn is the better caps almost always wind up being quite a bit bigger. Physically getting them to fit might not be worth the trouble. So there's that to consider too.
Notice soldered to the boards in front of the big caps are 4 black things sticking up? Those are rectifier diodes. Newer/better/faster diodes will make a world of difference. My very first mod ever was a HEXFRED rectifier diode upgrade on my Aronov integrated amp. Deeper stage, liquid extension, blacker background. Could not believe. Best $8 I ever spent. But again, they have to fit. Had to bend the heat sinks on the new ones to get them to fit. Don't look pretty. Sounds beautiful though.
That's about it. Yes it really is that simple. Of course you can make it as hard as you want it to be. Or easy. You can also just clean it up, spritz a drop of lube on the volume knob, admire how good it still is after all these years, and screw the cover back on. Hard to go wrong either way.