On your parts diagram you show 2 shafts, which would fit the FS 36. As well I would like to change the trimmer head.

Stihl have a couple of options available as this is serial number dependant. For your machine you require: 4130 711 3200 - Drive Shaft for Stihl FS40, FS40C - 4130 711 3200 https://www.lsengineers.co.uk/catalog/product/view/id/17079 The heads compatible with your model are as below. 4004 710 2131 - Mowing Head Polycut 6-2 for Stihl FS38, FS40, FS50, FS50C-E Strimmers - OEM No. 4006 710 2134 https://www.lsengineers.co.uk/mowing-head-polycut-7-3-for-stihl-part-no-4006-710-2127.html 4004 710 2105 - Stihl Mowing Head AutoCut 11-2 for FS36 FS75 Brushcutters - 4004 710 2192 https://www.lsengineers.co.uk/autocut-11-2-stihl-strimmer-head.html