In 1988 they used a Kohler single cylinder and an Onan twin cylinder. Here is how you can tell. Go tohttps://lookup3.toro.com/partdex/index.cfm?xCaller=Toro
In the green bar click on Advanced Search.
Next page 2nd box select Riding Products
3rd box select 1988
8th box select 16hp
Then click on Submit in the top right corner.
That will bring up all the Toro products in 1988 with 16hp which is 4 models.
Now click on Product details for each one to see the engines used.
Toro bought Wheel Horse in 1986 and it took them a few years before they changed the name from Wheel Horse to Toro so we left the 1st box set to all to make sure we got them all. If you have 1986 product or older you can safely use Wheel Horse in the search.
The B1-xxxx model is likely and export model from the US to Europe or Australia.