Micro CoGen.

"not just simply fogging the gas into the intake"
Too true, I convert vehicles to LPG and have done several diesels - post '01 diesels (in Aus') require a computer controlled management system and they are recommended for hard working vehicles, the exhaust temp' probe that shuts off the LPG over 600C is worth it's weight in gold, but we're not talking about road trains and those systems are pricey.

For pre '01 vehicles and gensets I'm happy to fog the gas in through a small orifice, about 3 to 5mm and have it powered via a throttle switch or turbo boost switch so that vapour only flows when the engine is starting to work, not at idle.
The orifice sounds tiny but it is easily checked by running the engine up to 2800-3,000 rpm and then switching the gas on, you are looking for the rev's to rise by 200-250 rpm

At this flow rate you will use one litre of LPG to every three of diesel and no modifications will be required, it will act as an accellerant and burn up the normally wasted diesel - no more black smoke, more power and, after two or three oil changes, no more black sump oil

Cheers
Rob
http://nakedmechanic.blogspot.com/

May or may not be relevant but - Hopped up flatheads with 6v starters would be OK if engaged with 6v by the key start and then hit with 12v by a relayed push button while winding - it stopped them from banging themselves to death
Your mileage may vary
Rob
http://nakedmechanic.blogspot.com/

Some random thoughts on this thread -

a propane fuelled engine does not need a choke and will idle comfortably within seconds of startup

use the smallest carb' that will run your expected max' load, much easier for a governer to control, look at forklift carb's and Impco 125s or 225s

do use a good thermostat to prevent cold running and fouling, I use a 160o for non-computer road engines, you may like to go higher for your heat application

a propane fuelled engine should not carbon or gum up and there will be less bore wear as there is no wet fuel washing away the oil -but- the oil will gradually become more acidic even though it looks fine, do not neglect regular oil changes or your bearings will suffer

a cam built for towing applications will have less overlap and max' torque under 2000 rpm, both are desirable characteristics for propane and woodgas

Cheers
Rob
http://nakedmechanic.blogspot.com/

A quick note on head gaskets - when riding and servicing 50's motor bikes in a previous life we made our own head gaskets from copper sheet, they were re-usable many times if annealed between use - just heat them to cherry red with a torch and let them cool naturally