TY175 6 speed gearbox, I would think should fit all 5 speed bikes? The cases look exactly the same. As the gears themselves aren't as strong as the 5 speed ones, then maybe the rubber damped clutch of the TY would also be required?

TY motor is from the complete motor revamp in "74 from what i remember, so couldn't see it fitting CT1-2-3 motors. But maybe i'm off the case & you're only all talking "74 on.

I never seriously looked at the TY175 because of my assumption the gear ratios would not be suitable for "racing"...  nor did I realize they had a 6 speed.  Plus a TY175 is very rare and parts quite expensive due to the "cult following" they have.

The basic TY175 architecture looks the same as the 74-76 DT175.  But the TY175 appears to have the same changes that the other 1976 six speeds received.  So trans "gears" may physically fit in 74-75 DT100, DT175, and non-USA "kick start only" DT125 (often designated an "F" model outside the US) there may be some work required to fit a six speed into a 74-on 5 speed kick start...  most obviously they went from a 6304 input bearing on the 5 speeds to a 6303 on the six speeds.  A big plus is that a 1976 six-speed model shift drum might fit the 74-76 DT175 five-speed cases?

RT325 is correct about the earlier CT and electric start AT and DT125's.  While the again the "gears" will physically fit, you have the input bearing issue, shimming issues, and no factory six-speed shift drum that fits the cases.

It's not completely "impossible" to convert to a six-speed...  but it's not just a matter of swapping parts either...  you will almost certainly need a six-speed clutch and matching crank gear...  primary ratio changed on most 76 models so that means a different crank gear...  Yamaha changed crank gear mounting over the years (splined to keyed) so that may mean a crank swap...  all depending on what model 5 speed you're starting with. 

The TY175 motor was, according to B&J Racing in the US, is based on the DT100, so I would imagine parts from the TY would fit other motors related to the DT?

Without modification, though, the TY can't really use many of its 6 gears in competition.

Like the TLR200 Honda, these bikes were designed to be very much beginner-friendly, and have very soft gentle power (and not much of it!)

In modified form it's a completely different story, and if the work has been done correctly, they are probably better than any twin-shock Fantic, and are able to pull 4th gear in the sections

Fitting an RD250 reed block, 26mm carb, and making a custom air box, if done together makes a great deal of difference. They don't seem to work very well without an electronic ignition, though, and that seems to be a must have?

Porting changes to increase transfer flow, are likely to work well on all pre 175MX motors, and mean more power at all engine speeds, with no reduction in reliability.

I have not tried one, but i would think a Vitos Blaster piston, with modified crown to increase port duration, would also work well on all 66mm reed motors?

One more thought... if someone wanted to upgrade a 74-76 DT175 to a six speed and get rid of that goofy left side shift lever linkage (not one of Yamaha's better ideas IMHO)... I'm pretty sure a later 78-81 "monoshock" DT175 engine would drop right into the 74-76 DT175 frame without too much trouble. Probably much easier than trying to shoehorn a six speed into a 74-76 five speed case.

Or for a real boost... maybe use a 77-79 IT175 engine? I might have to try that sometime.

Blaster motor would probably be better, and easier to find? I think they can be converted to 230cc as well?

I believe with the Blaster the head hits the frame if using stock mounts...  it's a taller engine.   But it can be done.  

Blaster pistons work in the CT/DT175 reed engines if you trim the skirt slightly.

Ring end gap location should always be checked though... on some 175 models, notably the IT175, the location of the end gaps is different enough that you can't use a DT piston in the IT safely.

I know someone who has a 230cc Blaster motor in a TY175 chassis. Have used Blaster pistons in TY motors, but no real point in this, as the boost windows do not correspond with inlet ports on the older motor.

The best piston of all for 175cc trials motors, is OE Yamaha, with Dykes top ring. They are getting very hard to find now though, so I use DT175MX, which are cheap and easily available.