my EXP C172 has about 100 trouble free hours so far....Its a GM engine block but a lot of mods to make it work safely for aircraft, especially in the internal oil system. AS far as man hours it took, don't really know but it was over a 3 year period of working on it part time pus another year of research and design figuring/head scratching. Weight increase was about 150 lbs but I added additional items as well. I would not recommend trying to duplicate such an undertaking unless you have a lot of free time, aero structural & electrical engineering background, as well as handy with welder, CNC machines and are a car engine guru..... even then it will cost you a fortune and would be cheaper just to rebuild the original motor and upgrade the avionics...with lots left over for new paint, upholstery and another C172. I get a lot of pilots asking about the V8 conversion and think its as easy as making a mount and bolting on a junk yard engine, its not if done right. Problem is there are not any such complete engines designed to meet normal cert standards, most just put together from automotive parts. And with electrical motors the new rage, not sure the expense of designing, manufacturing and testing/certification one would be prudent.
There are many auto conversions out there, and lots of accidents in them. Some because of the actual engine, but most due to the PSRU, or the car engine electronic controller / EFI system which is a poor choice for a plane even with "re-tuning" using commercial software. I tried this method but found car controllers are designed to prevent people from changing anti theft & emissions features and often open up other issues when you do. I've seen a number of auto conversions reverted back to carb and distributor set-up, but then there is no room for any meaningful redundancy. There are also some simple EFI systems marketed to exp aircraft, with mixed reviews but I have no first hand knowledge of them.
There is a company actually currently flight testing C172/182's V6 and/or V8's for STC certification designed to meet FAA cert standards. I only got a glance at engine about 6 months ago while they were putting the top cowl on, but it looked like a real production aircraft engine.... I think it was a V8 or V6. It had a neat LED engine display that monitored the engine and would alert if something was out of whack. It was extremely quiet when it taxied out and took off. They didn't really want to answer any questions about it, but stated the engine would not likely be available in US due to liability issue. Maybe they will make an experimental version for the US which may keep the price in check... but this usually isn't the case when it comes to aviation engines.
The cost of converting a C172 to the Continental Diesel is about $100k and it still cost about the same or more to operate as the original engine. It's no wonder why the experimental category is growing so much faster than the certified. A neighbor asked what kind of plane to buy, a C172 or PA28 (as if there was no other practical option)....I suggested finding a used experimental.