How hard is it to replace the NSS in a 1984 300D?

Mine is about to go. Only starts in "P" about 20% of the time. Will still start in "N" most every time without issue but I need to replace it soon.

Thanks in advance for any help.

PS -- Got a busted header too. How hard is that to replace? Can't get a good patch on the flexible pipe.

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'84 300D Turbo Black w/Tan (104k)
'94 GMC K1500 Z-71 (198k)
'96 Ford Explorer V8 AWD (178k)
'51 Ford 8N ( about 2000hrs)
'98 English Setter "Jade" (10k foot weary, bird finding miles)