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Welcome.
I'd stick to the same video driver transistor as original for now and maybe increase heat sink size if practical*....those old metal can transistors don't always die of overheat. My 71 Zenith hybrid uses the same metal can style of transistor to drive the CRT cathodes with color signal....on mine it was intermittently loosing a color for over a year before failure of the transistor...A slap or any attempt at voltage measurement troubleshooting would cure it till it died.... I'm pretty sure that's fine whiskers.
* Remember most of those metal can transistors the can is connected to one of the leads so make sure the heat sink don't touch anything that could short the transistor.
I know this is basic, but did you clean the contacts of the tuner? A dirty tuner can wreak all kinds of havoc....IIRC some 70s sets had issues with tuner fets shorting...If you have a transistor tester those may be worth checking. I doubt you have access to a tuner subber or a B&K TV analyst (which have IF outputs you could use to confirm the IF is working (don't mess with it unless you know it is dead or you have a working TV sweep marker generator and want to learn the dark art of TV IF alignment)...if your video modulator happens to be a blonder tongue AM series or something with an IF jumper often that IF is a standard 44MHz TV IF signal you could inject for troubleshooting.
Good luck.
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