Am I correct in believing a DT175 cylinder will work on a CT2 crankcase?  If so this appears to be a cheaper alternative than boring one.

Depends on what year DT175 you are talking about.  Also not sure how it could be cheaper than boring?

The aftermarket cylinder, piston kits ebay are pretty cheap. No idea about quality. But I only see them for DT 175. 

I've never seen one for an early DT175.  CT2/CT3 top end can be swapped with cylinder and head from a 74-75 DT175.  (A 1976 DT175 cylinder and head can also be used except it doesn't have the oil injection port in the cylinder so you'd have to use a 1976 carb or add an oil fitting to your carb.)  This design has 4 long studs that hold both cylinder and head.  

There wasn't a 1977 year model US DT175.  The US 1978 DT175 was the monoshock and had an updated engine design with the cylinder held to the case with 4 short studs and the head to the cylinder with 6 short studs.  (Design based on the 1977 IT175, but DT175 cylinder is NOT interchangeable with IT175 version)

There are also questions about if some of those cheap kits even fit the 78-81 and the quality is suspect, especially the pistons.  There was another Yamaha design change in 1982 for some countries (Last US DT175 was 1981) and the cylinder/head was changed again.  Someone here bought one of those ebay kits and it didn't fit the US 1978-81 DT175.  

That's great information Mark! I recently saw a 70 DT 250 on FB market place and asked the guy his lowest price and pointed out there was a couple of things things not correct for the year, also that it was in fairly rough shape. He ashed what I thought was wrong so I let him know, he then got all butt hurt and said he was a shop owner for 30 years and knows more about these bikes than anyone I know, I just laughed to myself and thought, he's never been on this site with people like you and 4 or 5 others that I'd put up your knowledge against his any day! Thanks for all your help and willingness to share your knowledge with us...

Thanks for the info guys! I'll just get it bored, no problem.