Oscilloscope Repair - Heathkit
I don't see much point in shotgun replacement of parts. Unless you have the ability to troubleshoot (as in another oscilloscope, and good schematic and the necessary skill set) you're asking for trouble. 

If the scope was not used much, it's quite possible the tubes are in fine working condition. 

Replace the cap that is sizzling. If it is left running, the transformer will probably be toast and then the unit is garbage. You should also replace the electrolytic capacitors (the metal cans and any others you can identify). This may be the schematic. 

Then test the unit as best you can and see if it will meet specifications. 

Be careful, the second anode voltage on the CRT is potentially lethal when powered. After removing power, make sure you discharge the high voltage filter caps and electrolytics before touching it or you might get a nasty jolt. 

They're not great scopes (very low frequency limit, AC coupled), but any scope is better than none.