Embarrassed to say one low side and one high side on my fxs in 4 months. Damage to a mirrors first time and a hand guard second time. Otherwise bike is spotless besides scratches on foot pegs. Jacket and helmet scuffed up but still good. A few bruises only, thankfully. Both times picked up bike and got going again. Pretty impressive.
I would get a fxs front rim and match the tires. You might want to tweak the front suspension to compensate for the smaller wheel. Steering will feel different also.
So I have had my fxs for about four months. I had no choice but to get the modular. My apartment does not have outlets in the garage so I went with two quicks. I lug the two batteries upstairs when they need charging. I live in the city and use my big bike for longer rides.
So far I'm very happy with this setup. Not to mention how great the fxs is in the city. I get about 40 to 45 miles on a charge and with the two chargers can get full charge in 2 hours 45 minutes. With a busy work schedule I typically charge up on a Friday night and depending on the weekend one or two more times during the weekend. It's been working great. Can't be happier having spent the extra money on charging.
I haven't considered the chain conversion so can't help you there.
New here. Thought I would share my experience. Took my 2016 fxs in for service today. The firmware was updated to v51. Was 49 before that.
Should probably add that I didn't notice any particular changes in the way the bike rides after the update, other than to say it seemed more smooth. Probably just my imagination but at least it seems to be more positive as opposed to negative.