A 50cc saw is pretty much maxed out with a 20" bar. I doubt you would be happy with the performance of 50cc with a 28" bar.
Could it pull a slightly shorter 24" bar with a skip chain? Most likely if you need it to, but you may need to stop for a piss break before the cut is finished.
If it were me, I would run the 50cc saw with the 20" bar and save the $$ you plan to spend on a longer bar. Look for an Echo CS-590 with a 24" bar. They can be found on eBay for around $300-$350 new, under $300 used.
For what it's worth, I had a 28" bar on my 74cc saw. I just backed it down to a 24" bar. Not because the saw couldn't pull it, but the 28" bar was often more than I needed and would actually become a hazard sticking 7" out of the other side of the log. I had to be careful not to hit whatever was on the other side of the log - the big rock, the ground, the other log, etc. The saw also balances better with the 24" bar.
ETA: Up the oil. Milling is hard on a saw. Having an under powered saw multiplies the wear and tear. I don't mill, but I think the guys that do run no less oil than 32:1.