The 26 was re-engineered into the 31. The fuselage is the same, the Mi motor drops in and bolts up to the 26 fuselage without any structural or mechanical changes at all. Only the wiring and fuel system has to be upgraded to match the induction system. To fit an engine into the originally engineless 25 was, I'm sure, a much larger job. I don't think converting an engineless 25 into a 25Mi is practical, without purchasing a new fuselage. I believe it has been tried on a 22, the cost is prohibitive.

On Sunday, October 4, 2020 at 8:39:22 AM UTC-7, kinsell wrote:
Obviously it wasn't trivial, but look at what they did with the '25.
Sold as a straight glider, a sustainer with a Solo engine, and a
self-launch with a carbureted Wankel, an injected Wankel, and a twin
jet. So much re-engineering went into that ship, but not the '26.
On 10/3/20 9:59 AM, jfitch wrote:
Dave, there was no good reason to do it. The E engine was running fine, had 45 hours on it, and it still running today in another glider. A number of 26E owners over the years have expressed interest in this conversion. We had the engine and were curious about AS claim that this was not a trivial endeavor. Also, I was going to need to replace the Technoflug prop at some point, and the Mi engine already had that prop. It turned into a bit more work than we (Rex and I) had imagined - perhaps AS was right - but the result is a nice upgrade. The alternative way to get an Mi engine is to spend the $1/4M dollar bill for a 31.

On Friday, October 2, 2020 at 5:41:57 PM UTC-7, wrote:
On Friday, October 2, 2020 at 12:46:57 PM UTC-4, jfitch wrote:
While the Mi engine is better, the E engine is certainly adequate
Thanks Jfitch for showing us your beautiful 26mi at the convention.
Can you remind us why you decided to do the conversion from original engine?

Thanks,
Best Regards, Dave