Yes, a 13 or 15 front sprockets will work. I've been using a 13T front sprocket with the various rear sprockets throughout the year (50, 55, 60). I've ordered my 13T front sprocket either from eBay or Amazon I can't remember which, listed for a TW.
It came with a spring clip
instead of the bolt retainer you see on the stock 14T sprocket.
You can get a 14T from Rocky Mtn ATV/Motorcycle for $10.74 (3 in stock). If you order it today you'll probably get it tomorrow. 99% of the parts I order from them I get the next day unless I order on a Friday or Saturday.
The closest I could get to how long of a chain was using this calculator which only went as high as 13/65 = 152 links. I suspect you'll need more so you'll probably have to buy two chains and cut & splice to get the links you'll want.
Sprocket Calculator - the easy motorcycle sprocket and chain calculator
I've heard of at least 2 forum members who've used a 70 or 72T rear sprocket but I can't find any info using the forum or google search functions.