The Allis 900 series was built by Simplicity (actually owned by Allis Chalmers at that time), and it's twin sisters are the Simplicity 7100 series tractors. All parts and attachments are interchangeable between the two. Older front attachments (your blower is a '60's model if it was originally yellow) can also be interchangeable, and adapted with the correct hitches. I'm guessing he has that conversion done...a picture would easily tell. Rear attachments will work just fine with the right belt length. Mid mowers require a pulley conersion for proper speed, and there are some lifted height clearance issues if an older deck is used on a newer machine...but folks make this work all the time.
912 was available in hydrostat and in '6-Speed' configurations. Sounds like you have a 6-speed on your hands. It's a very reliable setup. Simplicity made that cast iron, adjustable slip differential transmission, in-house, and with little modification from 1959-2000. Hydrostats use a modified version that takes power from a Sunstrand belt driven pump.
The 6-speed is basically a belt driven 3 speed (plus reverse) gear shift transmission with foot clutch, that has a second set of pulleys that provides a low range of a little less than half speed in each gear. It's a pretty bullet proof design. The idler/tension pulley on the drive belt simply shifts from one pulley set to the other when shifted from high to low. No planetary gears etc to care for or worry about.
GROUND SPEED - SIX SPEED TRANSMISSION
Gear: Lo-Range / Hi-Range
First: 0.7 mph / 1.6 mph
Second:1.7 mph / 3.7 mph
Third: 2.8 mph / 6.1 mph
These tractors did have optional electric power lift, and were Kohler K series powered. A rough one without power lift and with only a deck in good running condition should run in the $150-350 range. They are quite common and not as popular as the hydros. I wouldn't give the blower a whole lot of value...$50. The hitch (if correct), however, can fetch up to $100 when the snow flies.