The plans I found on
https://www.kensiron.com/ The actual frame and hammer can depend on what material is to hand .The important part is the control mechanism that links the foot lever to the control valve and hammer position .
There is no drawing for this so I copied the photo on the web site and it works as it should , I have had to make some adjustments to it as I could not find a suitable large bore air valve and ended up having to modified a 5 port 3 position hydraulic valve instead of a 5 port 2 position valve The main frame is 12 mm plate the base is 30 mm plate the anvil base is 8 pieces of 25 mm x 300 mm bar cut to 600 mm long .The hammer rod is 1 meter of 50 x 60 bright bar and runs in 4 pieces of oil impregnated nylon plate . the hammer guide frame is all made in 25 mm thick plate. Total weight is about 800 kgs or 1750 lbs .
I still have to make some interchangeable hammer heads and hammer blocks for the anvil but need to source some large pieces of work hardening steel .
It is possible to go from cracking an egg to full blows without resetting the control valve
I will not bolt the hammer down but will secure it to some cut up railway sleepers and let it move around
I like the design over mechanical hammers as it is far more controllable, quieter and delivers full power when used at slow speed.
I have built the hammer to assist in my other project which is the restoration of a large water mill dating from 1790
The picture is of a drive gear that is beyond repair that I need to replace