This vehicle would be considered an Assembled Vehicle in Texas, for which there are forms and procedures described in the Assembled and Reconstructed Vehicle Manual (ARVM).
Transportation Code, Section 731.001
(1) “Assembled motorcycle” means a motorcycle, as defined by Section 541.201, that is built or assembled by a hobbyist.
Transportation Code, Section 541.201
(9) “Motorcycle” means a motor vehicle, other than a tractor or moped, that is equipped with a rider's saddle and designed to have when propelled not more than three wheels on the ground.
An assembled motorcycle is one that is assembled from two component parts, a motor and frame, and may only be constructed by a hobbyist. The frame number is required to be shown as the VIN on all motorcycles assembled from 1971 to current. If an identifying number was not placed on the frame by the manufacturer or if the manufacturer's identification number has been removed or altered, an assigned VIN must be obtained.
An application for Texas title on an assembled motorcycle must include the following:
Approval letter/checklist from the TxDMV RSC;
Application for Texas Title and/or Registration (Form 130-U);
Government-issued photo ID;
Evidence of Ownership:
Frame - Bill of sale, Component Part(s) Bill of Sale (Form VTR-63), Invoice/Receipts for material used, Certificate of Origin, title, or photocopy of title (if titled in applicant’s name);
Motor – Bill of sale, Component Part(s) Bill of Sale (Form VTR-63), or photocopy of title (if titled in applicant’s name);
Rebuilt Vehicle Statement (Form VTR-61);
Vehicle Inspection Report (if applying for registration that is not custom vehicle/street rod)
VIN verification in the form of one of the following:
Law Enforcement Identification Number Inspection (Form VTR-68-A); or
If an assigned/reassigned number is obtained, the Law Enforcement Identification Number Inspection (Form VTR-68-A) and Notice of Assigned or Reassigned Identification Number (Form VTR-68-N). The original Form VTR-68-A will remain with the TxDMV RSC, and a copy will be submitted to the county.
Photographs of the front, rear, and side; and
Certified weight certificate.
At first glance, a reverse trike is a motorcycle as it has less than four wheels. However, with the introduction of the Polaris Slingshot several years ago, there are new "autocycle" rules for vehicles with 3 wheels but standard seats instead of the saddle of a motorcycle.
The ARVM does not include the term "autocycle"
Per the ARVM, "An assembled motor vehicle, replica, custom vehicle, street rod, and kit car must pass an Automotive Service Excellence (ASE) inspection by a Certified Master Automobile Technician to be eligible for title", but that does not include assembled motorcycles.
Does "Autocycle" mean not-motorcycle for the ARVM?
This could affect the overall design of the vehicle. The extra requirements for a vehicle vs motorcycle aren't onerous, but there may be a hole in the law and procedure for an "assembled autocycle".
Section 502.005 - Registration of Autocycle(a) In this section, "autocycle" means a motor vehicle, other than a tractor, that is: (1) designed to have when propelled not more than three wheels on the ground; (2) equipped with seat belts and a steering mechanism ; (3) equipped with seating that does not require the operator to straddle or sit astride the seat; and (4) manufactured and certified to comply with federal safety requirements for a motorcycle. (b) For purposes of registering a vehicle under this chapter, an autocycle is considered to be a motorcycle.
Will need to get a VIN assigned, I believe through "Law Enforcement Identification Number Inspection (Form VTR-68)"
"An assembled motorcycle includes a frame and motor. Ownership evidence for the frame may include a bill of sale, receipts for material used, a title, a photocopy of the title if the applicant uses the frame from a vehicle they own, or a Certificate of Origin. Ownership evidence for the motor may include a bill of sale or photocopy of title if the applicant uses the motor from a vehicle they own"
The "engine" could be the oil burner package, or the cylinder assembly itself (which would allow titling before steam is installed and the vehicle is air-powered). Perhaps the cylinder assembly could be built, "sold" to a friend, then "sold" back with bill of sale to fulfill that requirement.