The 220 conversion is a nice one. It is lighter than cont. or lyc. of similar hp. There a more around than one would think. The Stinson becomes a serious performer with over 200 hp. There are stcs for many engines.
The buzz at this point is that the polish company PZL has or is re-procuring the rights to the piston engine from pratt and whitney.
They have stated that if they do they will make parts for the older ones as well, things like crankshafts and valves.
There are lots of spare parts for the 165 still around, but cranks are becoming hard to find.
There is a business in texas ran by Susan Prall and her daughter or daughters i am not sure if it is one or two, Franklin Engine Parts. These people have put an enormous of money and energy to getting other spares approved. They have been successful on many parts and gaskets.
The key to a franklin is to have someone who knows them work on them. They have a few quirks, ie the cylinders are lined. They are also designed for no lead, and running 100LL can be a problem with sticking valves if the egt is not kept in the proper range.
There are several very good repair facilities for the franklin, and rebuilds are not bad, in the 8-10k range. THe TBO is 1200 hr. I think, but many are going well past that mark into the hight 1000s if the engine was fitted with a oil filter at the last rebuild.
If you go to buy a franklin powered airplane with 165 hp engine one needs to check the case numbers to ensure that they are "heavy case" castings.
The point of this is that the franklin is a fine engine, it is powerful, smooth and is easy to maintain, but, you need to be aware of its quirks and keep an eye on them just as any engine. Education and knowing your power plant and its operating parameters will help all brands work longer and with out as many problems.
Speedbump is correct, it is something to think about and get all of the facts and make an informed decision. This advice should be used for ALL ac and engines.
Oh yea, I forgot to mention that there are 108s out there with an odd ball military engine in them, it was an early conversion. This engine is orphaned and very hard to get parts for, and there is only one propeller that fits this engine, very expensive and has many ads on it.
If I can find the engine designation I will post it. My advice is to avoid this combination.