On something of that breed have you had success with welding it up?
Despite what one might see on u-tube or at the car-fair, welding of cast iron is always going to be iffy, if only due to the inconsistency of what constitutes "gray cast iron"; and although one with well developed technique will have better luck than others, the only proper response would be "no guarantees" whether in the survivability on the initial welding from cracking, machineability, not necessarily only of the filler material but also of the heat effected zones adjacent the repair, not to mention depending on the impurities within the casting its' appearance, and of course even if all is well to this point then the residual stresses that may lead to failure at a later date.
Best to avoid any plan that involves the necessity of welding work of complex castings made up of "gray cast iron"!
And again, even with experience, although it definitely helps, sometimes the casting just was not going to lend its' self to the installation of seat inserts, no matter who does it, it's just a chance one takes in the attempt of salvaging old shyt! Best to avoid this process unless really necessary.
Scott.