Start with an 8 foot beam, cut it in half and lay them parallel. Find some 2″x 2″ or larger angle iron and lay it on top and inside of the beams. Set the Lister on top, and trace the holes. Drill a 9/16s hole in the angle and inset bolts up thru the angle. Use a wood bit to countersink a hole deep enough for the bolt head, so the angle lays flush. put lag bolts in three places along the lower inside of the plate.
Set the Lister over the mounting bolts, note how the engine is set back to one end of the frame, with plenty of angle in front for the generator base to ‘track’ on. leave engine loose for the moment.
Note the detail on the Generator frame base, this is a few pieces of scrap angle welded to some flat iron to make a sliding mount. In this picture you can see that the generator is trapped and can not twist.
Here’s an excellent way to mock up the generator base and make sure you have the generator head in line with the flywheel. In this picture you see a very rigid piece of steel that is flat against the inside of the flywheel, and also flat against the end bell. You will also note that the pulley is opposite the the position so the belt is further from the operators hands.
In the pictures below, you see a bar across the front used as a spreader, this was placed to assure the frame is parallel and it also serves as a place to tie in the tensioner for the belt.
This is all too easy, and there is plenty of room to individualize your generator set.