I would suggest the Jett BSE FIRE-90L. Same size and bold pattern as an OS 61 or 91FX. Weight with the AAC sleeve is about as good as it gets at only 20 oz
If you want a screaming engine and full pipe, this is the way to go. The engine is also available as a side exhaust.
Having said that........
That application appears to really want a heavy prop. Maybe a scale type 3 blade prop. In that case, I would recommend the Jett 120L. It is the same size and only 1 once more than the 90L FIRE (weighs 21 as a FIRE-120L). It is not designed to scream...... runs around 10000 rpm, but will turn a lot of prop, and does not need a highly tuned pipe to do it.
Another option, if you
really want to save weight.....and don't need a lot of RPM....... look at the BSE 76L. Similar to the Jett 65L noted above, AND the sames size as the 50 dave originally mentioned, this has more power than most 60 engines..... will turn a 14x6 at 10000 rpm..... but is only the size of an OS 46 !! Weight for the FIRE version is only 13 oz ! Again, not a screamer engine.... has a practicle RPM range of 9500 to 12000 rpm range, and does not typically use a tuned exhaust (same is true for the 120L), so there is great variety in prop selection. (I currently have a 12x6 3 blade running on mine to gather some data, and the engine turns that quite well). If the original of this plane used an ST61 or something like that, definately look at the 76L.
For the 65L and .76L, there are some nice carbon pipes, used for control line stunt versions of these engines, that might be suitable for this application.
Bob