What they said, plus, what, nobody mentioned the ever-popular "Swellman" glow plugs?
Yes. DISCONNECT GROUND (seriously consider installing a keyed ground disconnect switch - details here somewhere) ANYTIME you work on electrics. I got lucky, mine ran, but blew a LOT of white smoke (unburned fuel), replaced my S3 (PCB) box 2 years ago, got it on sale for $475. Bought a "spare" a couple months ago (S3 - newer, bestest unit) for $400 delivered, but you'll most often see them for upwards of $700 now. A commonly replaced, easily destroyed vital piece of hardware.
GROUND everything. Instructions for a ground wire harness to bond engine, S3/PCB, frame, starter, battery are here somewhere.
Seriously consider pulling glowplugs once you're sure engine is running. I say do it AFTER because sometimes it's a real PITA to pull one or more of them as they swell, bend, get stuck, and as much as it's an exercise in good old-fashioned anglo-saxon and germanic linguistic gymnastics (i.e. - cursing) and more than a little skin and blood loss, there IS a tool for this if you run into problems and sometimes pieces break off which require pulling the injector at that cylinder to retrieve the broken piece(s) which I understand is GREAT fun (and more cursing), so best to see if the thing will run at all and how well it starts in cold weather before considering glow plugs. I did mine in that order. Glow plugs (tested, two were open circuit - dead), ran a little better afterward, THEN PCB. Glad I did, it was a chore getting the front left one out, broke the stud holding the generator to the cylinder head in the process. Bought the removal tool for insurance.
Batteries?
Not happy with Optimas, although I'm heavily invested in them. Bought yellow tops for my HMMWV, one died a year later, amazon refunded $$$, but price had gone up another $100. In for a penny, in for a pounding, I bought another.
THEN I got a pair for my MEP-802a. Recently discovered they'd both died despite being connected to a pulsetech solar charger, so I put the cheapo lawn tractor batteries back in and they're fine. The Optimas died after not running the genset for about 5 weeks. I charged them up on a 4A charger, kept 'em on trickle charge for about a week, tested them on a Snap-On carbon pile unit, both failed, one miserably, less than 9V left, other about 10. Friend (AOR here on the board) hooked 'em up to an Optima super-duper charger for a couple days, charger says they're fine, carbon pile tester agrees. I don't trust 'em, haven't advertised on fakebook marketplace yet. Figure some Honda drivers might want them for cheap.
Right now it's the RED tops military is using the their GENERATORS, and the VERY expensive 6TL batteries of one flavor or another for HMMWV, NOT Optimas. Some people have great luck with Optimas. One of the guys here on the board has had an 18+ year success with his and he lives in Alaska. Sadly, the case with many, many others is more typical of mine.
I've seen reports here about getting interstate battery equivalents (and other mfrs) for under $200 each, but I like having the extra space in the battery box with the smaller format batteries.
YMMV.
Oh, and buy the NEW style fuel level unit. While you've got the tank down, replace the braided hoses, the filler connector hose and cover gasket.
Have fun.
