Are the NG manifold holes smaller or larger than the holes in your old grill? I think bigger...if you want to play with fabricating your own, just get some tubing or pipe to match the old manifold, drill holes, cut old manifold, slip new tubes over the stubs you left of the old manifold, tack weld...get a propane regulator...
... if holes are smaller...enlarge them..
..if you need a new regulator, get one from one of those turkey deep fryers...it's adjustable and you can really crank you new grill to heat up fast using the adjustable regulator
Anything after the regulator is low pressure so you can cut and couple the hoses...
...yeah...so I'm crazy like that, but then I'm use to working with higher pressures
try 1200 PSIG!