The heatsink bar is not particularloy sensitive, but you do have to install it correctly. Can you image what would happen if you installed the spare tire on your car backwards? If the bar does not make good thermal contact, then the necessary cooling does not occur. MAybe a better analogy would be connecting the radiator hoses up wrong.
TO218 and TO3P are close to the same, and either would work. TO247 is a bit larger, but ought to fit on the Fenders. TIP142/147 also come in TO220 - DO NOT get those. All of those packages have bare backs and need insulation unless you find them in insulated versions. Me, I'd stick with the TO218s if that is what is in there now. Mouser usually has a good selection. Nothing wrong with Newark, I just never use them as a supplier.
Diode test on meters works the same on most all meters. If you google transistor testing you will find various tutorials. Basically we want to know the transitors three legs are not shorted together in any combination. I have never had the need to measure gain or anything, they pretty much are OK or they are bad. Oh sure, now and then I get something in between, but not usually when we are blowing fuses. The gist of it is the transistor acts like two diodes joined at the base pin.
CR29,30 get hot normally.
What to check? Look at the schematic, can you identify the TIP142 and 147? So what is connected to them? Between them are a couple 5 watt 0.47 ohm. They are certainly connected, and they should be checked for opens. DOn;t worry if your meter can;t measuer half an ohm very accurately, it can surely tell you if it is open or not.
Now see to the left of them the 8 diodes? They are all involved, just check them as diodes with your diode test. ANy shorted or open? And from that bunch of diodes are a couple resistor trailing upper left and lower left, R92-95. They are probably fine, but they are on the edge of the possible damage zone, so check them.
Now while google is hot, go look up Light Bulb Limiter. Very simple. Make one and use it.