According to Suburban the smallest of their "NT" line-up is no longer being made.
It was the closest match BTU and size-wise for the NT-22A like my 75 Safari. They recommend 1,000 BTU per foot of trailer length. Their representative recommended going up to the NT-30SP which is identically sized and only a few dollars more.
If you're replacing with the NT style furnace and want to keep the a bottom vent format, Suburban part# 520009 is a bottom vent adapter and you'll need 4 round covers for the side vents which are part# 050733. Both will run you about $20.
I found a source for the NT-30 on eBay Express for $399, much better than $600 elsewhere. I'll post more info about this vendor if everything goes smoothly. Right now I'm keeping my fingers crossed.