Replace a recoil spring on an astra cub in .22 short? Fixed assembly?

[C&Rtrader]

Try to replace a recoil spring in an Astra Cub in .22 Short. The recoil spring seems to be part of a fixed assembly? Anyone ever swap one out? Picture below of the current assembly and the spring I want to add\replace. Attached a PDF of the owners manual as well.

Thanks in advance

[AR1911]

3 years later....

I know the OP has moved on, but for future readers:

The end cap is threaded onto the rod and staked in place. Removal is difficult at best. Later versions had no endcap.
If you cannot remove the cap, the guide rod is easily made from rod stock.
Dimensions of my later pistol's guide rod:

OAL 53.65mm
Length under head 51.60mm
Diameter of rod 4.92mm
Diameter of head 7.26mm

These dimensions are not absolutely critical.