You can convert hydros to solids and it should not be too difficult. I talked about this with expert week ago and got useful first hand advice.
Just remove the high pressure assembly from lifter core (I have heard that it can be removed by hitting the lifter to something solid in a right way...), then remove the cylinder inside high pressure assembly. There is a small spring inside.
Basically you need to replace the inner cylinder and the spring with piece of solid steel that you then trim to correct leght to allow suitable valve lifter to camshaft clearance.




So it is the middle component on last pic you need to replace. I think it will have to be around 15 mm long and diameter should be as close as 10,80 mm as possible.
That is quite heavy chunk of metal (which we do not want) if it is solid so maybe steel tube is best bet. I think it does not have to be extremely high quality steel because it is not subject to high stresses. You still use same hardened part that contacts the valve tip.