There isn't a more conventional choice than a small Continental. These engines have powered many small aircraft for seventy-five years and most pilots have flown behind one during primary training. I decided to investigate the possibility of using a small Continental in the D7.
Three issues arise immediately when considering this engine:
A conversation with Robert Baslee, designer of the AA D7, explored how the Conti weight compared with the big VW. We determined that a non-electric O-200 would have similar flying weight to a VW that had a full complement of electrics and ignitions. Robert also assured me the D7 structure could be easily adapted to the Continental. Yes, the O-200 cylinders will protrude from the D7's cowl slightly more than the VW heads. We will need to address this issue once the cowl takes shape.
Prior to mounting the engine on the D7 for the final time I got out the uncalibrated game scales for some real-world numbers.
The scales indicated exactly 200 lbs with no oil or exhaust.
This does however include a lightweight starter but no alternator. The
Aeronca exhaust stacks weigh less than 3 lbs and four quarts of oil about 8
An alternator that mounts to the vacuum pump pad can be added for a penalty of about 3.5 lbs.
New Continentals are hideously expensive. A used engine was the only option I seriously considered so attention was directed to Barnstormers for available used engines. It became apparent that a used O-200 or one of its cousins could be purchased for about the same $$$$'s as a new VW with redrive. The hurdle that must be jumped is finding the right one, an engine with a good chance of having serviceable parts and history.