Hi ... is there any particular reason why you want to change the spring? I know that the square section jobbies have had bad press at times and it is true that they can create a rifle that is overpowered and with a harsh shot cycle. But I have seen examples of rifles fitted with a square section spring that are shooting nicely with no harshness. You may read that the square section spring mashes up the internals and scores the piston etc but personally I have not seen this happen ... I am not saying it could not happen, just that I have not seen it do so. I have seen an HW77 with a square section spring and an Eclipse and both were very sweet and I would not have know about the square section without looking.
As a replacement, you will find suggestions on 'that site' plus the airgun spares suppliers sites (Chambers / Knibbs) or even you may consider contacting one of the tuning houses for their recommendation. I seem to recall that a standard Airsporter spring does quite well.
There is a database on the forum and that suggests:
DIANA / ORIGINAL 45 21 15 3 270 Ox
DIANA / ORIGINAL 45 21 14.6 3.2 290 Versandhaus
The measurements are the od, id and wire diameter in mm. Ox is clear as a square section offering. Versandhaus is from a German Website called Versandhous Schneider (I think).
A BSA Airsporter spring will be very close to the Versandhaus spec ... look at the database for more suggestions from other rifles.
Titan springs are good but you tend to use a shorter spring that original equipment and this frequently means cutting a bought Titan down by a couple of coils to keep it legal.
But to return to the square section info ... as I said, I have seen some very sweet rifles fitted with square sections but I have to admit that if I am fitting a new spring I always go for the round section .......
Cheers, Phil