I was wondering what steel and tempering is needed to make a good sickle and scythe that can be peened.

we used M40 -en8 as forged air cooled.

Similar to 1040-1045 steel in the US. Sounds like a good choice.

to make a sickle forge stock to 3/4 +3/16 or 1/4 forge the tang , then cut your stock to 3/4 the lenght of the finished blade,then draw the last 1/4 to make the full lenghth keeping full thickness ,then bend on edge like turning a horse shoe into a 3/4 circle , then depending on whether you are making a left or right hand sickle ,for a right hand sickle lay your blank on the anvil with the tang on your right hand side ,then with a heavy cross pein forge the blade out towards you leaving the outer edge full thickness ,the 3/4 circle will open out as you forge and will turn into a sickle shape ,you will bumble about with the first few ,but you can get very smart at making them ,as quick a making horse shoes. Try with some mild steel first ,your biggest problem will be working steel so thin without burning it too hot ,or cracking it to cold. Go for it ,i loved making them