CC's of engine aren't important. Size and weight of bike and approximate shock mounting position* are.

Some of the shocks out there of the correct dimensions are for a 1000 lb Harley Cruiser... probably way too stiff.

There are still suitable NOS shocks out there (like Redwings, might have to be patient) and even companies that make brand new ones to order. If you're looking for something cheap, often you need to be patient. Newly manufactured custom high performance shocks aren't $100 anymore.... $100 in 1973 is equivalent to about $700 today.

You could try Hagon, Ikon, or maybe even Race Tech for really good new shocks.

*frames with "laydown" shocks or "forward mounted" shocks generally need stronger springs and damping than the conventional DT3 mounting.

I had Redwings on my 75 DT250 and recently switched to Progressive Suspension 12 series shocks and the heavy duty springs. Haven't ridden yet so don't know who well the new ones will work.

www.progressivesuspension.com/product/1485/12-series-dampers

www.progressivesuspension.com/product/1403/12-series-springs

 

I'm not sure that thread is still here but Mako used the Progressives on his Trans-America ride DT400 if I remember correctly and really liked them.

I think back in the day the company was called S&W which I used on my LT2 and made a world of difference.

Forgot all about the Progressives...  good option...  except many people want to pay 1970's prices...  I think my S&W's were around $40 or $50 a pair back then (with springs)...  so the relative price is about the same today. 

Highly recommend Ikon.

Had S&Ws on my Z1000. Made it slightly better, but concluded that the chewing gum ERW frame, was the main reason for the "interesting" handling..................lol

Very well-made shocks though, and a good choice today as long as not being produced in China.

NJB in the UK make some lovely shocks that other members have used.
I met Norman (the owner) at an autojumble where he was displaying and selling his shocks, nice guy and the quality looks very good.
Rear shocks for my DT3 (UK supplier) - Vintage Enduro Discussions (yamaha-enduros.com)

I stripped some NJB "Professional" shocks in the past, as I was interested to see what was inside. They had cast iron pistons......................lol

I used RD350 shocks on my Ct1c and they worked fine on the CT it was about a inch longer and set them on the soft setting sitting on the bike it was real close to the original shocks when sitting on it 

Anything shorter will be too short "in my opinion".
Also RD shocks fit on stumps with bigger id of the bush in the eye.
The shock eye is likely wider too, to give it more strength being mounted like that.
All a bit of a guess just thinking back to those days.
If they're good shocks you could modify the ends 'narrow down then' & fit sleeves in the eyes to bring size down to 10mm bolts.
Could even add an eye to an eye--stack on top & weld it to add to the overall length. Like cut an eye of your old shocks to use, to add. Sort of crazy stuff i do to save a dollar lol.
Ok maybe forget 'that'.